This isn't an easy one to answer, if only because some poeple are going to forget to look at these 2 pistols as the tools they are ... and get caught up in brand loyalty ...
Nothing wrong with that, but you're asking about owning a single one of them, and you want to minimize your "down time" if there's a problem.
Glocks are fine weapons, but you can hear about a lot of complaints regarding their reliability and functioning ... even among their "loyal" fan club. Recently, over on GlockTalk, there have been a few folks that have been sharing their frustrating experiences with their G30's being less than 100% reliable. A couple have even traded off to something else ... in one instance a G21, as a matter of fact. (Heavier slides have slower slide velocities, and are generally more forgiving of ammunition variance) All I can say is that one morning at our range, I was involved with a small agency qualifying their shooters with some brand new G30's, using Speer 200gr +P ammunition, and I watched 5 out of 6 of them experience malfunctions. Most appeared shooter induced ... Glocks are unforgiving if you use less than a very firm and stable grip, including a locked wrist ... and at least one malfunction was with a personally owned G30 that had "gizmos" attached, including a magazine extension that might've reduced the magazine tension below reliable operating tension with the higher pressure ammunition ... there's that slide velocity thing again, apparently combined with lessened magazine spring tension ...
Here's a link to the S&W forum, of all places, where you can read about what a L/E Glock armorer thinks about the G21's he's maintained for his dept for several years ... he's posted more than a single thread on this subject, but only when asked, as he's primarily interested in discussing his other pistols ... and he's NOT into starting Glock-bashing ... he's merely less than enchanted with the reliability of the G21's he's responsible for maintaining, and will respond when asked. His forum name is SWCQB 45, which will tell you his brand preference, but you might consider reading his 2 posts in this thread, anyway ... This isn't the first time he's expressed a certain amount of frustration with the "tolerance stack" phenomena of Glock parts replacement.
http://www.smith-wessonforum.com/ubb/Forum12/HTML/000950.html
G21's have reportedly always had a "weak" spot regarding their magazines. I think Glock is on their "4th generation" of magazines for this pistol, and the frame and slide have been redesigned since their first model. The 15 degree angle of the breech face where the extractor is located has apparently helped with ejection issues. Naturally, there have been numerous and ongoing changes to the magazine followers across the board with Glocks to help with reliable feeding. I watched a man that was using a G21 to qualify with for his CCW one evening have continuous feeding malfunctions with 1 of 2 magazines he'd brought ... every magazine load ... and it was a new one, at that.
Now, I've never had a Ruger P90 magazine malfunction with any ammunition. They may be more expensive than folks would like, and hold a couple less rounds ... but I think they'd feed rocks if they were shaped more or less like .45ACP rounds
G21's can be somewhat finicky feeders, depending on the bullet shape, according to many, many owners who post threads. It's baffling to me how some G21 owners can posts threads about problems with certain bullet shapes, and then other people post that theirs even feeds LSWC bullets. Why is there this claimed difference in what a single pistol design will reliably feed?
I fired quite a lot of 185gr Silver Tips through a T&E G21 one time, comparing it to a full size USP45, a couple of S&W's ... 4506, 4566 & 4563 ... and even a couple of 1911's. It functioned fine, but it was dead last when it came to comparing accuracy.
Rugers are "blocky & chunky" in some people's hands ... but if it fits yours, who cares? Ditto for G21's ...
I've owned a P90 since the DC models first came out. I have yet to find something the pistol won't reliably feed, chamber and fire. Even some really grungy 200gr LSWC reloads that were used for our qualifications something like 12-15 years ago. These loads were even roll crimped.
Ruger has a great "warranty", although they seldom refer to it as that ... They take care of their customers.
I've heard conflicting reports of owner satisfaction for their polymer P97 ... and I really, really disliked the one model I test fired one time ... Horrible trigger ...
But you'll have a hard time finding anyone that's dissatisfied with the reliablity and functioning of their P90. They may not like the "feel" of the grips, or the size ... but we can talk about the G21 that way, too ... so it's a "wash", in that respect.
If I had to give a potential new .45 owner any advice at all on making a choice of these 2 pistols ... I'd suggest they do a LOT of forum searching among the GT, S&W, Ruger & TFL forums for owner experiences.
And then buy the Ruger ...
You'll eventually buy at least one more .45 pistol ... but you might just find you'll keep the Ruger for many, many years ... and it'll function boringly reliable for those years ...