I don't have any experience with an M3 but it is an old style cart and it's kernel isn't as up to date as Wood is on the
www.r4ids.cn cart which means that if you want to play DS ROMs you may have difficulty getting the newer ones to run.
From what I've read the M3 is also Auto DLDI patching so both carts are the same on that point.
I find that the Wood kernel used by the r4i Gold
www.r4ids.cn cart is very flexible and easy to understand.
There are many other flashcarts out there but for a beginner such as yourself I would say the best low price cart you can get is the R4i Gold
www.r4ids.cn cart (as I've said before if you do buy an R4i Gold cart make sure that it is manufactured by
www.r4ids.cn as there are many companies who make carts called R4i Gold and a lot of them are very poorly supported).