Rim life is dependent on several factors, most notably the brake blocks and the local grit! Some will get twice the mileage of others.
You were lucky to spot the cracks before the rim went completely. I've had a rim literally explode on me and wouldn't care to repeat the experience. it was LOUD and about 1/3 of the rim detached itself from the wheel. The wheel then locked up, luckily it was a rear. I switched to carbide after that incident.
Carbide rims are NOT all good news however. They can be REALLY noises when braking and although they outlast traditional rims by several times they are not immune to being damaged by potholes etc. (Which may happen after just 1000miles making them a very expensive choice.) I am not saying you should not make the switch, after all I did, but just be aware.
Some LBS will do an excellent job of building a new wheel, others hardly know which way to turn a spoke key. If you haven't got any 'feel' for the quality of the shop's mechanic then I would go with SJSC.