While UGCs are the most popular choice simply because they've been around the longest, most places online plus a few of my friends generally agree that the Shadowfox cases are of much higher quality. That's not to say the UGCs are bad, but like I said Shadowfox cases are generally considered higher quality, build wise and also in terms of the amount that break in transit. Also a few other things to note:
1) NES games wont' fit at all in the UGCs without modification, and I've heard that the Genesis games are extremely loose.
2) The Shadowfox SNES and N64 cases aren't actually out, though the SNES cases are available to pre-order, and I have heard something about them both releasing simultaneously.
So yeah, I recommended going the Shadowfox route but ultimately it's up to you.