Both have reputations for high reliability (#1 requirement). Grip feel and such is your choice. You'll want night sights on whatever you choose; for some odd reason Ruger doesn't offer factory night sights, Glock does.
Every review I've read of the Ruger comments on its outstanding accuracy, but I have not seen a direct comparison to the G21.
Lots more holster and accessory makers cover the Glock line than Rugers.
Another oversight on Ruger's part: no accessory light rail. Does it matter? Your call.
The Ruger is 1.3 ounces lighter empty, and the difference will be greater when both are loaded. Magazine capaicty is different. Glock: double stack. 10+1 standard, high caps available but expensive. Ruger: Single stack 8+1. Older P90 7+1 mags will fit. Assuming you'll carry a spare mag in either case, if 17 rounds of .45ACP won't solve your problem, you're done anyway.
Do you prefer the Glock safe action trigger or the Ruger's DA or DAO choices?
Finally, at list price, the Ruger is about $200 less than the Glock. You can shop around, maybe find a deal on one or the other.
The standard advice is to rent both and try them before you decide. If I were trading up to a .45, though, it would be the Ruger, because except for caliber it 's the same gun as my Ruger P95. Nothing new to learn.