Pros and Cons of buying on-line vs in store vary by vendor.
Sometimes, on-line offers better prices than in store. For TMO, if you buy on line, they can charge it to your account but if you buy in store, must pay for when you get it.
In store you have it in your hands now; on-line you must wait until it ships. You don't always know it's on backorder when you place the order. Buy in store, if you have a problem, you can exchange it in store within first 14 days. Buy on line, you have to handle exchanges through customer care and wait for exchange to be sent to you.
With Verizon, one pro to buying online is that usually *not always* if there is a mail in rebate in store, this is offered as an "online discount" online, and instant, not mail in. Which means, less up front cost for you and no waiting around for a rebate to be returned to you.