Aball,
The two preamps are (not surprisingly) similar; articulate, very slightly rolled off, and throw a wide and deep sound stage. Not terribly tube-finicky, they sound damned good even with the stock tubes BAT provides.
The 30 is more flexible, has a cool display, and really nice remote. In terms of sound qualities I find the 30 is a bit more liquid, little bit better bottom-end, all-in-all somewhat more emotionally involving than the 3i. I could happily live with the 3i, but for the extra money I'd go with a 30.
BAT components are very well designed, quality fit and finish, excellent after sales service, and most importantly very musical. I'm not affiliated with BAT in any way, bought my preamps and BAT VK500 amp used here on the 'Gon. Happy listening, Jeff (artist formerly known as jeffloistarca)