Beetle, if you're flying with 1x20mm, you're best off firing both MG's and cannon rounds at the same time. On their own, neither of these weapons do much, but together they hit fairly hard. With 30's, a hit anywhere but the engine will tear it apart. The 30's are much better for a "quick kill", but their balistics are terrible, so really high angle deflection shots against a zeke or a spitV are even more difficult than with the 20's & MG's.
The 109's are not meant to be flown in a sissy-like manner in a close-to-even odds situation. And they're definitely not meant to be flown fast, fast being in excess of 400mph. They're E fighters which are also capable of TnBing the faster, less maneuverable planes, but that's just a waste of it's strengths. Yoyo's, ropes, and hammerheads are the main things you need to get good at to be a decent 109 pilot (other things too, but those are the things you'll most likely do more often).