I'll second Tobers above. Wi-fi availability is the exception rather than the rule at stadiums, and requires a lot of infrastructure that most clubs aren't prepared to put in. New build stadiums are excellent of course (Wembley, Emirates etc), but don't expect it at your average ground.
Wired positions pitch-side are also rare.
You will pick up BT OpenZone here and there and if you're willing to pay that can be useful.
It is sensible to carry two or more 3G dongles by different operators as their coverage can vary dramatically. At grounds with 25K plus crowds you can struggle at times when the network capacity bottlenecks - half time and full time etc - and if you combine that with poor coverage it can be really difficult at specific locations (I always struggle at West Ham for some reason).
This can also change your wiring behaviours just to get a picture out. If its really chugging I will just send the best frame and keep the file size/pixel dimensions down; we don't always have such freedom of course, but worth considering. Then lump the rest through when it calms down.
I would say unless you've got an absolute pearler, don't give so much attention to getting the image out or you'll miss the rest of the game. I've spent 80 minutes before fighting to get images out and unless its a cracker that few people got, just leave the computer and get on with the business end of the job.