If you’re going to be gigging on a regular basis, the weight (or rather the lack thereof) of your amp should be a primary consideration unless you already have an small entourage of technicians helping you move equipment around. For medium-sized venues, a 30-watt amp should be more than sufficient as far as volume goes. The one thing to look out for is that most of the amps that you find will be open back combos. Tube amps don’t do too well in a closed cabinet due to heat issues but for metal you’re probably going to want a closed back cabinet.
This is a hard segment to buy an amp in because of the huge number of variations of amps. There are some however that stand out in terms of a unit that can do everything, so here are some great amps built for gigging in the $300 – 600 range category.