Let's take your advantage of Powerbolt, force 5, and assume people roll average.
You cast Powerbolt, with 12 dice. Your average is 4 hits, and the average to resist with, say, 4 defense dice, for 1 hit. That's 3 net hits.
Those net hits apply only to the damage you deal to the other guy. Not to your drain! So you do 5 base damage (because force 5) + 3 net hits, for 8 damage. Unlike guns, this isn't further soaked by armor.
Then comes the drain resist. The drain value for powerbolt is (F/2)+1, so 5/2+1. Drain value is rounded down in SR4, so 3 drain. Drain only depends on the force of the spell. Since you have 9 drain resist dice, you roll 3 on average, so on average you take no drain (though, of course, you'll roll badly sometimes).
Net result is that the enemy takes an average of 8 damage, while you occasionally take 1 or 2 stun damage if you roll badly.
I think the confusion here comes from the abbreviation DV, which stands both for Damage Value and Drain Value. The Combat Spells section mentions that net hits increase the DV, but that refers only to Damage Value, not drain value (it's listed under the Damage Value subheader, after all).
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Now, guns will have similar results to this - you'll get on average about the same net hits, DV will be higher but guns get an armor roll on top of that. And yes, guns have the advantage of shooting twice. But you have the advantage of options - you can choose to overcast at force 10, greatly increasing the damage you take but almost guaranteeing the opponents immediate demise. And you have other spells available. Plus, your listed dicepools are lowish - not terrible for a starting character, but certainly something you'll increase later on. As stated, getting your Drain Resist up (get the Centering metamagic!) will help a lot, since you'll be able to use higher force spells with less danger to yourself.