G3 (470-505 Mhz) , J1 (554-590 Mhz), and M1 (662-698 Mhz)
Some of the frequency bands I have come across in looking for bodypacks for lav mics.
The differences and issues depending on location or situation?
Any preferences to brand or model?
It really does not make a difference - just make sure you get something with plenty of diversity. I prefer the Lectrosonics wireless sets - I've never had a problem finding open frequencies. They are also very expensive. You can get away with using the Sennheiser G2 series without too many problems - just not as durable or reliable.
The reason they have changed from the traditional 700MHz range in the US is the recent shutdown of analog television stations. It used to be an open frequency, but is nos reserved for emergency communication. If you find a great deal on a mic that is in the 700 range - DO NOT BUY IT. Not only is it not a good idea, soundwise, but it is also illegal to operate. Wireless mic manufaturers are operating in the proper ranges now and will not sell you a pack that is illegal or running in high traffic. They only sell those ranges in case you have a theater or venue with dozens of mics and have potential for overlap - lets you break up the sets of mics a bit better. In the field, you never really know what you'll run across.
What frequency you pick is generally determined by the area you'll be working in. Check out this link and enter your zip code to see what frequencies are already in use for the area in questions: