SkullBiscuit, a TD is designed for ambush tactics. If any tank can flank a TD and track it, the TD is then pretty much dead. However, if a TD can get a shot it, it sure can pack a lot of hurt. The reason the Marder II kicks ass is because it gets put in more "even" matches, which is mostly lights and mediums, with maybe a heavy or two. So you can get at least 3-4 kills easy with the Marder. When you start with the Stug and JgPz IV, you are now in a totally different league and it will take at least a JagdPanther to be competitive in this new grouping. Sure, you may get "even" matches with the Stug and JgPz IV, but they are rare and far between, thus making you feel they suck. I'm very confident that with all things being equal, the Stug and JgPz IV are better than the Marder, it's just that with the way the matchmaker works, it throws those two tanks in very bad matches.
While I do agree that a clearer way of awarding XP is nice, it would be very hard as I'm guessing this is based on a lot of factors such as the tier of the attacking tank vs. target tank, the amount of damage inflicted by the attacking tank vs damage done by allies, crew skill level, etc. etc. Things can get confusing very fast.
Pooch, you can definitely have fun without spending any (or too much) money, but you'll have to be realistic with your expectations. I've pretty much played up to Marder II level without spending any money and was very fun. If you're happy to play the lower-level tanks, then that is entirely possible. However, if you want to play with the "big boys," expect a very tough grind.
Vitesse, what mid-tier tanks are you talking about? I've had fun with most mid-tier TDs and arty I've played, which is mostly German TDs, lights, and arty.