what drill is stronger?
a 7amp electric drill or an 18v rechargeable drill...like lets says 18v ion dewalt
also, is it easier to screw in screws onto hard wood with an impact drill, or normal drill
I have to use both types of drills daily, and your question is just a little tricky to answer. First of all, if you only want one drill, a good quality electric with a 1/2" key-less chuck is the only choice I'd advise anyone to spend their hard-earned cash on. You'll always have power to spare, and never a dead battery.
As for the 18 volt drill/driver, they will drive that screw just as effectively as the electric, but having the battery run out of juice half way through a job is VERY annoying. That's why I have 4, and they are ALWAYS charged. Because cordless drills (the good ones) have a gear reduction option, they give GREAT control when you need it, and that's priceless when you do. If the drill doesn't come with a "fast-charger", it's probably junk. If you're really set on buying an 18v drill/driver anyway, consider the Ryobi. I own several, and compared to every other brand that is with us on job sites, these offer the average guy the biggest bang for the buck.
As previously mentioned by the other fella, hammer drills are used exclusively with carbide-tipped masonry bits for masonry/brick/mortar/tile type jobs. Try to drive a screw with a hammer drill, and you'll never keep the driver's bit in the screw!