Best Buy no MIR (I guess none at Verizon too), larger selection of cases and accesories. Best Buy still has a machine that can transfer contacts and photos (e.g. if you're not coming from a prior google phone w/ backed up contacts). If you want protection best buy has a 9.99 a month option, covers accidental damage, battery replacement, hardware issues (but not theft) no deductible while verizon's insurance is cheaper, has a deductible. Free enrollment right now with the buy back program.
Cons: limited stock, generally not sent as many units as verizon (highly suggest you pre-order), some people just hate best buy and refuse to shop there. some accessories might be more expensive
As far as porting goes, just bring in your girlfriends bill or account number and best buy will have no problems porting over her sprint number. just make sure she is fully out of contract vs eligible for an upgrade. Otherwise she will pay an etf with sprint.