I have an early 21SF with a pic rail. I was surprised to learn that it is kind of rare now as Glock has been actively trying to replace the frames and the ambi mag catch is a weak link.

So do I sell it and get a different 21SF or just have Glock change out the frame? It hasn't given me any problems yet.

If you use it much the mag catch will break.  

I'd see about getting the frame replaced.

Personally, I'd just keep using it until (when/if) the frame breaks and get it replaced then... unless Glock puts a cut-off window on the replacements of course.

I would keep it or sell it. Peeps who collect Glock will want a 21 w/pic rail even though they break.
Getting where there are fewer of them. I wouldn't mind having one to add to my collection.

But,, for the present, I'm only buying what I do not have in terms of models. Doing .357  now.

Up to you. What do you use it for. My mags would stick bad in mine. Plus finding holsters was a little more difficult. Glock replaced my frame. Should have done the whole gun. They even made me pay to ship their defective mag dropping frame gun back to them. Talk about CS. That one still irritates me.

Jeremy is right about the holsters.  You'll have to custom order.  There are few companies out there with the option on their website, but not many.

I bought pic rail 21sf used and initially the mags would only snag very infrequently, but before long it was very difficult to seat a fully loaded mag, or get a fully loaded mag to release.  If the mags had 11 rounds or less, it worked fine, but full was a problem.

Another thing to consider is how many mags do you have and do you plan on buying more?  Because the ambi mag release uses a lever different from the standard mag release, there is an extra cut out in the front face of magazine.  I learned this the hard way when I bought extra mags for my pic rail 21sf and none of them would seat.  

I send the entire gun back to Glock and yeah it kind of sucks to pay the shipping, but they replaced the frame with a new gen 3 sf frame and replaced every other part in the slide other than the barrel and sights.  From the time I shipped it until I got it back was right at 10 or 11 days.  Now all my mags work and seating and releasing full mags is just like every other Glock.

I'm kind of in the same boat as you.  I bought the first 21sf I put my hands on. Never gave the ambi or rail a thought until it came time to find a holster - UGH!

For me it's just a range toy- way too big to carry.  I don't shoot it that much. I doubt I have much over 500 rounds through it in five or so years.

But it's never given me any trouble inserting or dropping loaded/unloaded mags. And I've got plenty of mags.

I'm afraid if I return it I'll have a mismatched number "franken" Glock. I've got a 2nd gen 17 like that with a replaced 3rd gen lower. I would assume that reduces the resale value - although I highly doubt I'd ever sell anyway.