i love the m1a/m14. it has been my favorite MBR(308) for a long time,some of that is because of my love of wood and history(where it comes from),..up until more recent. i now prefer the HK(real HK=$$) or an excellent clone(PTR,real HK tooling and drawings from portugal). why?
1) price. you can get a great basic rifle at a manageable price that is very accurate. plus,the receivers are in spec and many use it for their builds. even the welds on these rifles are often better than on real HKs. you can even splurge to put a RCM hammer forged barrel on,with everthing correct,and still come out ahead($) especially when you consider the cost of accessories like mags,etc..
2) modularity. many of the parts on HKs can work across the board with other HKs,stocks,HGs,mounts,etc.,etc.. this means you can create something really interesting down the road if you want or need,..and most of the parts to do this are relatively inexpensive. you can even have a PTR modded for drums and if you have a registered sear,FA. remember,the g3 was adopted by many countries and there're different configurations and different calibers. the M14 was basically adopted by USA only(mentioned in reference to modularity). PTR also makes a pistol version if you like.(SBR anyone?)
3) sights. many will say the m1a/m14 has the best sights out there,i won't argue with this,it's a matter of preference really,..but have you actually used them ever? there's one thing i don't find the best with them,identifying targets in a real possible situation. the HK drum sight is different but,imo,great at 400m and under,it has a 100m notch that is really fast for close up and for low light conditions. the apertures also create a really clear sight picture. you have to learn how they're used but once you got that down,they're great. mounting optics is not a problem on a m1a/m14,but on the HKs it's a pleasure imo. you can use the HK claw mount or you can buy the version with rail already there(the rail is as low as can be,so with the right optic and/or rings,you can get the height you want,including co-witness if you like).(you can still get an original hensoldt scope for about $400,not bad considering the quality and history).
4) reliabitity. i'm not going to come here and say the m1a/m14 is unreliable,i know better than that,but i will post this link and talk about the HKs.
http://www.youtube.com/watch?v=z0-3OQsnR-w
the HK roller delayed locking system is a beatiful thing and simple with no gas system but with high precision and quality execution. it is very tolerant to dirt and muck and the flutes are kind of self-cleaning up to a point as long as the flutes are properly done. generally,powder burn rate is also not an issue as it can be with the m1a because it doesn't use gas to power the action. with the PTR91,just don't use tar sealed ammo and if you need to use any ammo reliably,get the GI version. also,read the manufacturers warning on what ammo not to use and compare with others.
the triggers on the stock PTRs is not great but you can get a trigger-job from triggerwork.net if you cant do it yourself,..and then you'll have a great trigger.
i use to think the ergonomics stunk on these HKs but not any more. off hand sweeps the handle back as it moves to the mag,remove replace mag,off hand "HK slaps" on it's way back to HG. easy peasy.
your call,but it's just another opinion and my 2 cents. both are good systems,you can't loose either way with good working rifles.
you can also get a PTR91 MSG perimeter sniper rifle,compare yourself the price,specs and parts used to the high end M1As.
good luck and stay safe.