Best prepaid option for short US trip?

I'm visiting the US for 5 nights soon and would like to use my Nexus 4 without facing a large bill from my UK network, and so I'd love to know if purchasing a prepaid SIM when I'm in the US might be a more sensible option. I've never got a prepaid SIM abroad before so I'm not sure how it would work, but am I right in thinking I can buy certain prepaid SIM's from places like Walmart and use them 'out of the box' in my N4 without any annoying set-up process? Are most prepaid SIM's available in Micro-SIM format or would I need to take my SIM cutter?

If so, what are some of the better options? I've heard about Ultra Mobile, and I also gather T-Mobile themselves have a 5GB plan available from Walmart. Any other good deals at the moment? My main use would be data (checking Google Maps and whatnot), and I'd like at least 500MB of data, though I'm not bothered about lots of call/text allowance.

Any help is much appreciated. Cheers!