There is a noticeable difference as you move up in the range. Novo is a superb "budget" headphone amplifier, but the Solo offers what I can only describe as a more organised sound with better soundstaging with the majority of headphones we've tried them with.
As with many things, try and listen before you buy. The big advantage of going for the Solo Intro is the upgrade path to the Solo SRG II simply by upgrading the power supply.