im also a 17 year old with limited income
..i would like to upgrade my 10 gallon and my 40 gallon ... any advice?
It sounds like the best bet for you is Hagen's Aquaclear line. They are probably the most popular, simplest, cheapest, and best all-round filter.
I'd reccomend getting the biggest filter you can afford, so for your 10 gallon an aquaclear 20 is appropriate, 30 is better, and for your 40 gallon an aquaclear 110 is best, but a 70 would do the job nicely.
You can get the lower ones and it will work better still than an sponge or undergravel filter, but the price difference isn't that much in my experience. I think it's about $5 to upgrade from an AC 20 to an AC 30.
edit : spelling