I’d go ahead and get a 12×18 if I had no barriers. If you’re comfortable with a machine of that size, I think it’ll totally be worthwhile. The printing area and strength of the press will be nice to have around, even if it is a giant beast of a machine. 🙂
I have a C&P 8 x 12 that I use everyday and a 12 x 18 that I use about once a month – I say for sure go for the larger one so you can do 11 x 17 posters. Otherwise it’s not worth having one barely larger.