Several guys on here favor certain brands. I buy according to gallons, physical size, and price. I also look for efficiency rating and warranty.
You would do well to get a 50 gallon water heater. If electric it's nice to have one with two 4500 heating elements. That's enough heat to give it a pretty good recovery time so you aren't waiting all day for the water to heat up again after a bath or load of clothes. If you get a 30 gal with only one 3500 watt heating element you will be taking alot of cold showers. For the little bit of difference in price it is well worth getting a good big one. The only other consideration is making sure it will fit in the space where the old one is....measuring tapes help figure that out. I would also recommend getting the flexible supply lines to hook it up with... it will make installation so much easier and professional looking.