Applying for ESTA while in possession of a valid B1/B2 visa

B-1 is the visa type and immigration status of a business visitor. B-2 is the visa type and immigration status of a tourist or other visitor for "pleasure." The suggestion that using your dual-purpose B-1/B-2 visa for a different purpose could somehow cause problems is ludicrous at best.

Don't apply for ESTA. Use your visa.

Is there a clear difference between traveling on my current B1/B2 visa as a tourist, and applying for ESTA instead? Which one is preferable?

As you may be aware, the visa waiver program allows visits of up to 90 days, while those entering with a visa are generally given six months. VWP travelers also waive the right to judicial review of any decision to refuse entry or remove them from the US. In general, it's preferable to travel with a visa, but the differences will be significant in only a small proportion of cases.

While my B1/B2 visa is still valid, am I actually allowed to apply for ESTA?

It's not entirely clear, because the statute and regulations do not contemplate a situation in which someone who has a visa would ask for the visa requirement to be waived.

Some of my friends suggested it's not ideal that I reuse my visa for this purpose (saying that being rejected at the border when showing my visa would make me ineligible for another visa or ESTA for life).

You are just as likely to be refused entry with ESTA than with your visa. The criteria for inadmissibility are the same. (In fact, you are negligibly more likely to be refused with ESTA, because in that case you must not only be admissible but you must also qualify for the VWP.)

The relevant eligibility question is:

Have you ever been denied a U.S. visa you applied for with your current or previous passport, or have you ever been refused admission to the United States or withdrawn your application for admission at a U.S. port of entry?

You would have to answer "yes" to this question after being refused entry, regardless of whether you had been seeking entry under the VWP or with a visa. Statutorily, you are only disqualified from using the VWP if you violated the terms of a previous VWP admission (8 USC 1187(a)(7)). The relevant regulations (8 CFR 217.2(b)(2)) note that persons previously removed or deported from the US require "consent of the Attorney General" to apply for admission, and excludes such people from using the VWP. But most people who are refused admission are allowed to "withdraw their application for admission." This is legally distinct from removal or deportation, as noted at 8 CFR 235.4. For VWP travelers, the relevant regulation is 8 CFR 217.4(a)(3): "Refusal of admission under paragraph (a)(1) of this section shall not constitute removal for purposes of the Act."

Having said that, however, the ESTA algorithm is notably opaque. Someone answering "yes" to the eligibility question quoted above could be denied ESTA authorization even if the facts behind the affirmative answer do not trigger any automatic grounds of ineligibility. In that case, the prospective traveler would need to get a visa.

But this is no more likely to happen if you try to enter with your visa than it is if you try to enter with ESTA, and, given what you have presented in the question, it is exceedingly unlikely in your case.