It really depends on your location and how much you travel. It seems like the AT&T speeds are better because there are fewer people on their LTE network. Of course that won't help you if you live in an area that only has LTE on Verizon. I went with Verizon because I have AT&T on my phone so hopefully I won't ever be without data.
Will the AT&T 3G sim in a Verizon iPad loophole be closed? I am using the AT&T sim from my iPad 1 and my plan auto renews in a few hours. It is the older $25/2gb plan and I might as well keep it as there is no LTE within 190 miles