Now that I look back, I didn't do a good job answering your question. Given only the two figures to work with, I'd say rpm, as long as they also give the prop used. For example, some will say, "Our 46 will turn 18,000 rpm." They do not say that this test was done at 10 degrees Farenheit, with 30% nitro, and a 8x8 El Cheapo prop. To me, that number is BUNK.
With real information like, "Our 46 turns a 10x6 prop at 16,000 rpm on 10% nitro..." I can work with that and make a guess that an 11x5 will also run near te 16,000 mark and a 12,25x3.75 or 12x4 will also run somehwere in the 14,000 to 15,000 rpm range with similar fuel and atmosphere.
Companies are trying to sell engines, so they try to make it look as good as possible. A 46 size engine can range in BHP easily from 1 to 2.25 horses. That number is derived in near perfect situations with undersize props to let the engine scream to achieve a high number. Small prop BHP is meaningless when you look at 3D planes or even good aerobatic planes. The number we need, is the number they won't tell us. We need to look at the torque of the engine. That is what will hepl determine if the engine will, for sure, crank the oversize props that we use. If it runs a 10x6 at 16,000rpm, will it turn the 12x4 at 14,000... Only the torque (or experience) will tell.
Like I said, however, it's best to come up with a plan, and just ask around. It's also good to ask about weight (incl a muffler), as some manufacturers don't seem to know that all of the time either.
I hope this helps,