There are three types of magazine styles available for AEG rifles. Not every AEG has the option of all three but the popular designs generally do. Using the most common terminology these can be described as:
Low Cap - Usually 30 rounders - matching the "real steel" firearm's standard magazine capacity
Mid Cap - Usually between 75 and 190 rounds - Spring Driven, needs to be loaded with a speedloader
High Cap - usually between 200 and 600 rounds - loaded via a loading hatch, requires to be wound up to ensure it will feed.
There are pro's and con's to each which I'll cover in the same order.
Low Caps - these usually have the real steel capacity so if you want to Milsim it up or you just fancy a challenge these are for you - you'll probably not ever want to fire full auto or you'll be reaching for a new mag instantly. The Pros - at the end of the day with airsoft we're simulating combat and it doesn't get more real than lets say, if carrying 9 bombed 30 rounders - only having 270 rounds deployable without reaching for a speedloader. Your mags won't have loose ammo in them (as the spring tension system in the mag stops this) meaning you won't rattle as you run. The Con's are quite obvious too - you're having to carry a lot of magazines and you are not getting a lot of capacity for your effort - that said - you'll probably grin from ear to ear for every kill you manage - as it will be a mean feat especially if squaring up to players using either of the other two options.
Mid Caps are now creeping into a few manufacturers AEG boxes as the standard issue - G&P immediately spring to mind for this. These mags are very popular with milsim players and a lot of experienced players tend to prefer them too - but why? The midcap will on average carry between 75 and 120 rounds, with up to 160 and even 190 in certain cases. This is several more times over the real steel weapons they mimick - but this difference is often "waived" as in real firearms where you would be able to put a single round on a target for an effect - in airsoft you will probably fire a few more - so the increase is usually seen to balance this back out again. The Pro's - you'll still be reaching for a new mag pretty frequently which adds the element of ammo awareness and reload's into your airsoft gaming. The other advantage shared with low caps is that the spring system maintains full tension throughout all times meaning the magazine will rarely, if ever rattle and give your position away. The Cons? You'll need to carry more, you'll need to reload more, and you'll also need a speedloader to reload your mags.
Most manufacturers include 1 or 2 high capacity magazines when you buy a gun new, and most sites issue 1 high cap magazine - so these are pretty commonly found in the field. The main advantage to these is quite obviously their capacity - it allows the user to carry a lot of ammunition thats "good to go" and do so without carrying too many mags. Having two full high caps will usually mean you are carrying at least 400 rounds (in the case of the MP5) or up to 1200 (AK etc). This is a lot of firepower and you often know when you are being pinned down by a high cap user - the rounds will keep coming and coming as they will be a long time between reloads! The con's of these magazines are also worth considering - first of all because of the capacity you don't get to reload very often - which if you were soldiering for real you'd need to. Secondly, the high capacity will probably mean you find you burn through considerably more ammo and thus your day will end up more expensive. You also suffer from the position indicator that is the dreaded high cap rattle - as soon as you fill and wind the magazine there is a space inside the magazine for the BBs to rattle around - and as soon as you move quickly - they will - thus telling your opponents where you are! Whoops. Not a problem for some, but elimanates any thoughts of sneaking around! Finally as high caps aren't spring tension based you need to either (depending on the magazine type) wind them from the underside to keep them feeding or pull a chord to keep the magazine feeding.
What is best for you can only be decided by one person - and that is yourself. You will know your playing style and you will know your budget and you'll know what is, and isn't important to you.
Obviously there is no requirement for you to be exactly one type of player - for example an increasing number of players are mid-high hybrids - they run a number of mid caps and reach for these first - but carry a high cap or two for those "Alamo" situations - where they need to put down supressive fire, or they've bled the mid-caps dry.
I've written about this before and been very honest about my preference and I'll do it again - I'm a mid cap user - and the reasons are very similar to what I've listed as the Pro's for the mid cap mags further above.
First of all, I can carry 5, 6 mags and have approximately 600 rounds ready to go which I find is nearly always enough in any one game. If I fear its not, I'll strap a few extra ones onto my rig before we set out. Second of all, I can sneak around all I want without giving my position away - and thirdly - and perhaps most importantly I can return fire against the "high cap heroes" who have dedicated an entire 600 round magazine to me by throwing it at the cover I'm crouched behind. I think in an ideal world I'd reach for low caps but the reality is - you simply cannot expect to rock out with 30 round mags when there are people on the opposing team with 600 per mag... its just not going to work.
In summary - have a think about how you want to play, and what you want to do with your loadout - only you can decide - but make the decision consciously - don't let the manufacturer of the gun you buy decide for you (by including either a mid or high cap!) Remember - if you have one type and want the other you will nearly always be able to sell what you have and pick up replacements in between two gamedays at your favourite site.
Last word on mid and low caps specifically is check out the HK, China etc retailers as they can often offer "boxes" of midcaps at very keen prices - sometimes converting from running an M4 with two high caps to running it with 8 mids can be accomplished for as little as £35 - and that leaves you two high caps to sell on to recoup some of that cost.