Depends on what you want to put in it, and how much work you want.
One of the reasons people will recommend getting as large a tank as possible is, the larger the volume of water you're working with, the smaller the effect of small chemical fluctuations -- that is, the larger the tank, the more stable the water will tend to be. Which usually means less work/worrying/problems.
Also, with a 2.5 gallon tank, not a lot of fish will "fit" in it, and those that do, you won't be able to keep a lot of. You're probably looking at two fish, maybe four tops, if they're very small, like neon tetras.
Personally, unless I was making a betta tank, I wouldn't bother with a 2.5.
/www.planetcatfish.com/catelog/com_list.htm#12 - at a list of a the 'L numbers'. There are many there, I am not sure whether they are all plecs or if there are more plecs that are not L no's though.
, personally I would go for the 10g which if more long than tall would allow you to have one of several species in there, remember though only one Pleco. CS mentioned the Zebra and Leopard Frog, both of which are small growing Plecos, there are also some others like the Clown Pleco which in my experience only reaches 3-4 inches and can fit in a long 10g but it would much rather a 20g so if you want other bottom dwellers skip the Clown. Another downside to Clowns is that they are socail fish and should be kept in groups, so I would actually rule these out.
