Angus,
Sorry to hear about your difficulties with the PE-90

...the only solution I can see is for you to ship yours to me right away! I'll even send along one of my AR-15s kicking around in my safe in return!
As a right hand shooter, I really haven't had to "notice" problems "Lefties" will have with the rifles. My classic Green and CQB are just about my favorite rifles I own. I was a little wary of "Plastic" magazines before using the first one, but I've come around. Being able to lock extra mags together makes the SIG/Swiss Arms system very convenient, moreso than the AR system.
I really don't have a problem rocking mags into place, have had to do it with the M-14 for years. Gravity drop mag system isn't high on my list of priorities since there's no way I'm going to drop a $70.00 magazine onto concrete or rocks at the range and when hunting, quick mag changes really aren't an issue unless you're hunting seals!
I don't shoot really heavy for caliber .224 bullets, pretty much stick with the 50 & 55gr V-Max or ball so faster barrel twist rates isn't an issue.
My only beef is the need for a brass deflector. Having the paint chipped off your $3200.00 rifle by ejected brass isn't fun for guys who are meticulous about their guns as I am. Don't like to do it, but I use tape or stick on velcro to protect my paint.
That said, I have to say I love both the SIG and AR platforms. Both have high "look cool factors", both are proven systems, in terms of quality and robustness, I think the nod has to go to the SIG, however, the modularity of the AR platform probably makes for a more useful, all-around, rifle system. The sheer amount of accessories, uppers, different barrel configuartions and relatively cheap magazines are certainly to its credit. However, in Canada, you also have that pesky problem of being RESTRICTED. That sure puts a crimp in things.
I love my SIG/Swiss Arms rifles, but I'm also in the process of building another AR-15...so you don't have to choose between the two, you can own the best of both worlds instead!