I recently transferred a bunch my old legendaries from previous games into ORAS using the poke bank and poke transporter. I'm looking to put some of them on the GTS but im not sure which versions are better and worth holding onto. (ex: entei, raiku, suicune, cobalion, terrakion, virizion, landorus, tornadus, thundurus)
Other than having the Blue Pentagon mark (The '6th Generation' mark), and possibly better Individual Values (IVs), there wouldn't be any difference (stat wise).
In terms of value however, both have positives:
- Previous generation Pokemon can:
- have moves that they can't learn in the current generation
- have Pokeball designs unavailable in the current generation
- have Ribbons that are unobtainable in the current generation
- be old 'limited time' special Event Pokemon (sometimes with special moves, like Flying Pikachu) which are harder to obtain.
- Current generation Pokemon:
- Are tournament-legal (Blue Pentagon)
- have your OT Name/ID, and are thus nicknameable
- Can be more easily farmed for better Nature/IVs
Thus both previous and current generation Pokemon have value, but to different people for different reasons. The former will be highly sought after by collectors, the latter by people looking to battle competitively.
If by better you mean, have higher stats in general, then it's likely to be yes, the legendaries you will get in the newer games would generally have higher stats than the previous ones, and that's because of the 6 IVs a legendary has, they are guaranteed to have 3 of them maxed out while this is not the case for legendaries from games before XY.
But I say 'generally' because some people soft reset a whole bunch of times to get legendaries with perfect, or at least close to perfect IVs, or some are just lucky and they get legendaries with really high IVs.
In your place, I would first see what are the legendaries' IVs and decide if their nature is fine before sending them off.
If you are looking for better IV'a then yes they would be better, aside from that I believe that there is not much else that would make them better/superior in any way because even if a new move was introduced a Pokemon transferred from a previous generation it should still be able to learn moves introduced in the new generation.