Is an open-ended ticket more practical than a one-way ticket for touring China?

Generally speaking, you will need to have proof of onward travel within the time limit set by your visa to enter China. Airlines are preferred, though I personally have never had issues at the overland Hong Kong crossings without proof of onward travel. But on the flip side, I know people who have, and where you enter the country makes a difference in this respect. If you buy a one way ticket but try to enter as a tourist (or on any other sort of limited stay visa), there's a good chance of being refused, including at your home airport when departing. You don't want to be in that position.

An open jaw ticket is more expensive than a planned round trip, but you're paying for the flexibility. You could consider booking a round trip ticket with a very late return date, and opting to pay the alteration fee to shift to an earlier date when you want to leave. You ought to also be able to change the departure city in this case (again, for a fee). Whether or not this is cheaper than an open jaw depends on your specific plans and how far in advance you make them. If you decide that you want to leave China the next day, then almost certainly the open jaw would have been the cheaper option. But deciding a month in advance, probably not.

It's worth noting that several of the major Chinese airlines will consider your ticket valid for one year. That's not to say that changing its dates will be free, but if you've bought a pricier fare and do it at least several days in advance, it might be. My answer on another question about Chinese airlines' cancellation policies is relevant, as many of them use the same policy for cancellation and alteration.

I doubt this is feasible, since you mention flying to Beijing, but for completeness' sake: if possible, I recommend using Hong Kong as your destination airport and traveling overland to China (or perhaps Taiwan and taking the ferry). You'll incur a few hours' travel time (2-3 from HKG to Shenzhen) and the price of transit (should be under 10 USD, assuming you take the metro) but the airfares using HKG instead of SZX or CAN are considerably cheaper in my experience. Overland travel is also much cheaper and generally more flexible than airfares.

I recently did something similar, and that included some Asian countries, including China. First hand experience:

Flexible tickets tend to be much more expensive than a round trip fixed fare.

I'm not sure which Airline you are flying, but Turkish Airlines, Kuwait Airlines, and Emirates are likely. All three Airliners have these flexible option, but in my opinion, flexible tickets are unreasonably expensive. Do a search and see if two round tickets are cheaper than the round trip.

If you plan to fly budget airlines in China, Malaysia, Taiwan, Singapore, Thailand, Indonesia, Philippines, or Vietnam

Don't forget to check Air Asia. They have many code names (AK, D7, etc), but overall, Air Asia is a wonderful budget Airline. Return tickets are priced same as two separate tickets (excet you'll have to pay for credit card processing). This wouldn't apply to countries that you can't manage to fly budget airlines for the entire trip.

Changing the flight date will require you to pay the pay difference anyway In most cases, when you change the flight date, if the new dates rate is higher than what you paid for, you will have to pay for the date change and the fare difference. This is practically more expensive than buying two tickets. Flexible tickets should be fine though.

Flexible Open Jaw tickets !?!?

Open Jaw tickets are expensive. Flexible tickets are expensive too. Flexible Open Jaw tickets are going to cost you even more.

I would suggest that you just purchase a ticket from origin to Beijing, and your very next trip first. I have never been asked to show a return flight ticket while entering China (We have to get visa in advance though), and I have never seen anyone being questioned. If you can convince that you really plan to leave the country and you have no strict plans, you should be fine though. This outwards ticket doesn't have to be a return flight to back home -- just a ticket to get out of the country.

You will probably don't want to go back to China either.

Beijing, Shang-hai, Xi'an, and these cities are huge cities, but a 2 month trip means you'll be overwhelmed with China. Language barrier, weather, smog, and such things are exciting at first, but in my own experience, I'm done with China for a few years :)

Cost? Flexible tickets tend to be more expensive.

Do your own research first. But I believe separate segmented tickets would give you the best flexibility vs price combination. Since you mentioned visiting other countries, China has an excellent network with Hong Kong (administration area), Taiwan, and Russia. For other countries, flying will save you money and time (From China, you can even enter India from the West Bengal region. But arranging transportation and getting used to India from there would be too difficult).

Visa requirements? We may need to present evidence that we actually plan to leave China.

Probably. If you plan to visit Taiwan or some neighboring country, you should be fine. Unused visa stamps for other countries work very well too.

Flexibility to change our plans. Are open tickets fixed to the departure airport? What about an open-jaw, open-date return flight?

I haven't really had a chance to book a flight or even see this open to book a ticket that origin of the return flight is flexible. This will definitely add up some extra charges if the Airline has to use some less-than-popular routes for you. If this actually worked, I'd consider myself very lucky.

Don't worry about the return dates first. I once spent a lot of money on return flights, skipped a few return flights, and even booked another flight ticket because I was lazy to take the train. Carry some extra cash, and do not be too quick to spend money on a ticket. If you have a valid visa, wait a little more and you'll find some perfect deals. Flexibility costs money, but if you manage the little details, you should be fine.

Enjoy your trip.

If you only want to buy a one-way ticket to China and worry about getting the other one-way ticket when you want to leave, buy both tickets now, but have the return trip be refundable. Then, when you get to China, you can show proof of onward travel at immigration, and then later at your hotel/hostel/wherever, you can just cancel it. Then you can buy a flight you actually want closer to your departure date.