At this point, T-Mobile has closed the network gap to the point where it's a wash for most people (which is the core reason why Verizon was pressured to intro a unlimited plan in the first place)... either carrier will get you what you want. So it's a matter of which gives you the better deal. Now, on a per-line bases, T-Mobile's One plan is still a better deal (in particular if you have two lines)... not only is the base charge less, T-Mobile now rolls taxes and fees into the price... So for 4 lines, T-Mobile's One plan is $160 flat, where Verizon's is $180 + taxes and fees.
Now, the tricky bits is current customers. Both are throwing stuff like free phones when you port in a number and trade in an eligible phone... or ETF reimbursement fees, etc... So for, say, a T-Mobile customer switching to Verizon, a free phone or two can alter the economics of the plan.
Competition works!