I'm on the other side of that. I have a B3-1000, 10 gallon system. I do a mix of 5gal and 10gal batches, pretty even so far. When I'm trying a new recipe I don't know if I want 10gal of it sitting around. I also love the process (don't we all) and so I could potentially have some go stale. I supply beer for poker nights and have parties, but I haven't done the club thing and so I need the flexibility to go lower if I need to.
My point is that If I do 5gal of a "smaller" beer, or if I do a 5gal batch with a decoction my mash level can drop below the thermometer, really just an inconvenience, but it is a problem. If you by a 20gal system you may have trouble doing 5gal on it if you want to. If you don't ever do less than 10gal it's not an issue.
Rob



