The 904 is more affordable. Many have done that and say it's worth every penny. My buddy Dave(roadworthy) has the 904(with other upgrades like head work, carb work) and it has a ton of torque of power.
I'm sure they'll chime in or do a search on 904 BB and you'll get a lot of previous posts on this topic. There is also a lot of dyno sheets on here you can look at too showing the diff between the stock motors and their performance after their upgrades. You can get a LOT of power out of a 904.
