As most people her have said it has a lot to do with budget. Saying that this is my personnal experience with these brands.
TM
When I started out they where pretty much the only company making AEG's. I have no experience of the new proline series but their stock weapons are the cornerstone of airsoft today and most AEG's are based on their basic concept. However they are normally plastic bodied and under powered, normally 300fps or under. They have reasonable attention to detail in apperance. Easy to modify and and upgrade, and if left stock will run forever on a 8.4v powerpack.
For a first gun, you can't go wrong.
As for their GBB pistols, they are plastic instead of metal but are amazing and as they are plastic tend to run all year round no matter ther weather on green gas.
ICS
A brilliant make of gun, full metal generally, with full metal gearbox, the armalite series has a split gearbox design which allows easy maintanence and allows you to swop the top section contain the spring (this is an advantage if you play different sites with different FPS). extremely reliable and i have never been able to brake one. I understand that the ICS SA80 is not made to the correct scale, also i have had problems in the past with aftermarket parts (body and stock) fitting correctly. As a result it can be difficult modify you gun in the future.
G&P
Without arguement the best looking guns out there, with amazing attention to detail and normally with all the right trades, but unfortunatly it stops there. Everyone of these i have owned has suffered with poor internals and generally have cost me a fortune in hops, pistonheads, gears and motors before i give up and get rid of them. always a disappointment. if your lucky you can get the 1 in a 100 that works out of the box for more than a few skirmishes.
Classic Army
Of late a good make all metal, with good internals to go with it, to good sometimes and they need to be downgraded to be within acceptable fps limits. aftermarket modifications fit without problems. not used there sportline range (cheaper guns) so i can't comment. A good all round make that will last and is eay to modify.
G&G
I have had nothing but problems with this make and no longer will purchase them, The M16 I owned had a misaligned gearbox from the factory and as such simply ripped itself apart every few hundred rounds. As an example of their poor quality and bad aftercare when they launched thier UMP is was so poor they were forced to include tappet plates (part of the gearbox) when you purchased additional mags. their reputation got so bad a lot of UK shops stopped selling them.
VFC
Simply Superb, great guns built to exact standards, with good quality internals. The guns tend to be able to field strip as per the Real Steel ones, as such Airsoft accessories may not always fit, where real seel ones might. Excellent guns but with a price ticket to match
Star/Aries
Starting off as an Accessories maker star launched it's first gun a few years ago. They are generally good gun but their product testing is poor. Almost all of the guns they release are riddled with problems to start with, once the guns been out for a while and Star/Aries is on the version 2 or 3, they are then worth buying. The nice thing about their guns is they tend not to me the normal M16-MP5 stuff. Good Make, Not as expensive as they used to be, don't buy any brand new release until it's proven and watch out for some poor 2nd hand models
Hope this helps