Both are great. Personally I like the smoothness and melody of the dobro more than the twang of the banjo, but mastering either will be a great addition to a bluegrass band. Another thing to consider is price – my gut instinct is that you can find a better quality cheap banjo versus a cheap dobro, and it’s also worth looking around in classifieds or on Craigslist, or even at the instrument shop itself (they often have a wall for classifieds) to see if anyone’s selling a used one.