At the end of the day any practice, even on a very subpar keyboard, is better than no practice.
However, there are differences in authenticity between digitals. The more authentic, the easier it is to transition to an acoustic. Look for fully weighted actions as unweighted or semi-weighted actions can give you bad habits over time. In my opinion the cheapest digital that makes the grade as far as being a good acoustic replacement is the Yamaha P155. It is the cheapest piano to feature the GH action, which I think is a big step up from the lower end GHS action.
I do all my practicing on a digital, and then perform on both a grand and uprights (just at church...I'm not a professional or anything). I have no problem whatsoever with the transition. In fact, I think my digital is more similar to the grand than the upright I usually play on is. Personally I play a Kawai MP8. It has their wood action and I like it, but I played the predecessor to the P155 before and it was good too.
Acoustic pianos are good too, but very loud and very heavy, and they are never in tune. Often they are in poor regulation as well. If you can afford a good one and have a good space in which to play and not bother anyone, they can be real nice.