Another thing to consider when you're starting out is how much are mags going to cost? If you stick with hicaps then you'll usually have just one with the gun and it's not that much of a stretch to buy another new at the same time and save yourself the P&P. Two hicaps will get you through any skirmish you're likely to get in as a noob. But at some point you may well get sick of winding them and the rattle they make, the remedy for which is to switch to midcaps and you may conceivably want to be carrying 6, 8, more even if you start going to all day events. So you may very well want to buy some 2nd hand...
...which is when how popular the gun is comes up - 2nd hand G3 mags are always quite a bit more expensive than M4, AK, or G36 and it's very rare to come across anyone selling Sig mags at all, unless as part of a package with the rifle.