Just got both Deoxys and Mew from their respective islands, and I don't know which to use, considering I'm avoiding using the same type twice. Main reason I ask is because I'm not sure which of the two is really better, but if Deoxys' Alternate Formes are able to be used, especially on a whim, it would be superior. I'm not really looking at the stats, more so the usefulness, but even if I was, I still couldn't make a proper decision. Deoxys is much higher on its Attack, Sp. Attack, and Speed, but is lacking in Defense and Sp. Defense. Mew, however, is pretty close to even, and has more health. Can't really make a proper decision without knowing what this game in particular allows...
If the Alternate Formes are able to be used, please let me know how to get them!