I think it really depends on what you want to do. I use
a Sansamp RBI and have been pleased with it for a long time. It's got a great range of sounds and a great distortion when used in moderation. The tone controls really do what you want them to do too.
However, I recently bought a Brent Averill 312a mic pre/DI and I am blown away by how badly this thing spanks the Sansamp on bass. It's so much punchier and thicker....it's a night and day difference literally.
Being that I have come to prefer simplicity when tracking bass, the 312a is going to render my Sansamp obsolete for the most part. It has opened my eyes (ears?) to how much better of a DI I can get from the bass. Unfortunately it's obviously a one trick pony...that might be a deal breaker for alot of people.
I don't know how the Avalon would compare, but I would imagine that for that one sound it would probably be worth it, but if you're after versatility you might be happier with the Sansamp.
Another thing that comes to mind for me is the positively huge range of sounds you can get from the Sansamp. To me that can be a bad thing because people want to fiddle with it and it's real easy to arrive at a sound that seems cool but totally screws you in the mix. With a good, solid and simple DI you can learn it and adjust to it and know what to expect.
Maybe I'm just getting tired of having too many options....don't listen to me