My first gun was an AEG & sold it promptly to buy a GBB. Many years have since gone by and have gone through many GBBs.
COST & WEIGHT
If you are considering that route, please keep in mind that the magazines can be very expensive (~£50 each), as well as weighty. Due to the low capacity, you will probably want about 4 - 6, depending how trigger happy you are. The price & weight definitely add up from the mags alone. Personally, I go for second hand packages that already come with mags for a greatly discounted price vs new, but buying second hand has it's cons.
MAINTAINENCE
Maintaining a GBB is not too hard - I would prefer to maintain a GBB as opposed to an AEG. As mentioned above, YouTube is your best friend. Keep the metal on metal contact areas oiled/greased & clean.
BRANDS
I heavily recommend sticking to TM & GHK. If on a budget, pick up the GHK G5. Otherwise, a TM MWS/MTR16 or a GHK M4 will do you good. Wouldn't recommend the GHK AK as their hop unit seems to have issues more often than not. There are other options available if you aren't feeling the M4, such as the GHK AUG or TM Type 89. TM & GHKs are built brilliantly which is often reflective of their price.
You will find other expensive GBBs from companies such as VFC - I recommend steering clear of them. Their price comes from their incredible externals and licensing, not their durability or performance (hell, my 416a5 doesn't even work & hasn't even seen a skirmish!).
WE Tech is budget friendly & they have a wide variety, but they are made of cheese, both inside and out. You will be on MilspecSolutions or KYAirsoft shopping for parts, which will eventually add up. I have owned many WEs due to their selection, but ends the same way.
There are many other brands offering GBBs, but again, I recommend TM or GHK.
GAS
Green gas is expensive, I recommend buying regular propane & keeping the gun & mags oiled.
ALTERNATIVES
TM & KWA offer recoil AEGs. The recoil isn't anything to write home about but it is a good compromise. Steer very clear of the GBLS DAS GDR15! Very cool concept, but they seem to spend more time with a tech trying to get them to work than they do with their owner, which is ridiculous considering the cost.
TL;DR
Mags are expensive & heavy.
Maintaining them isn't too hard, YouTube helps.
Stick to TM or GHK.