You can flash the 64 bios onto the 56, so that's why people are getting the 56.  I've personally got a 64, because it just seemed easier and I don't have as much time as I used to.  If you find it fun and you're not doing this because you have free electricity, you should go with whatever you've got time for.  My vega rig wasn't very hard to get set up to mine at 150 or so watts.  at over 10 hashes per watt (xmr), that's a lot better than my rx480 rig is doing.  And it has been easier to set up so far.