5Gs are usually just a little bit more money than 2.5's, and since you have a hood for the 5G, he might save money that way, if thats how the tanks are sold.
hoods with lights are nearly impossible to find for 2.5 gallon glass tanks.. atleast i have given up. i'd go for a 5 gallon since you already have the hood, and that's always the pricier part. also, with a 5 gallon you are a LITTLE less limited as to what you can keep in it.