I prefer Mega Evolutions over Cross-Gen Evolutions. With regular evolution, you can't go back to the previous stage, and in most cases, the evolution outclasses the pre-evo rather than just improving it. That usually leaves the pre-evo left aside in favor of its evolution. (i.e. Sneasel is cool, but rather weak. Perhaps an evolution will improve it. Enter Weavile. Sure, it's a massive improvement over Sneasel, but then it basically replaces Sneasel, since who would still use Sneasel over the faster, stronger Weavile? Now, instead of being improved, Sneasel is left behind and forgotten, outclassed by its evolved form.) Megas, on the other hand, provides its base form with a great boost, while still leaving the base form the same. Thus you don't lose your Pokemon in order to make it better. With regular evolution, you must choose to either leave the Pokemon the same without improving it (thus remaining underpowered), or evolve it to give it the boost it desperately needs at the cost of never seeing the pre-evo again. With Megas however, the Pokemon can get the boost it needs to kick butt and still remain as it is. Other than Eevee, I wouldn't mind if there were no more Cross-Gen Evolutions.
(There are exceptions like Chansey, which outclasses Blissey in Gen 5+ thanks to Eviolite)