I currently have a Spike's ST-22 4.5" dedicated 22 upper for one of my SBR'd lowers.  I like it ok, but really don't like the rail on it… a 7" YHM quad rail.  It's too bulky and I don't care much for 7" hand guards.  I shoot it suppressed and am pretty sure I can get a 9" rail on it, or the SLR 8" Solo.  I'm also looking at the BCM 9" KMR (if it ever comes back into stock).  Anyway, I started thinking that if I was going to spend that much money on just a handguard, that I might just go ahead and get a 15-22 instead.  My 22 upper has to share the lower with another lower so it would be nice to have just a dedicated 22 AR.

So, any positives or negatives to either one?  If I stay with the 22 upper, any suggestions on the rail?  Thanks

On the up side, the 15-22 magazine system is very reliable. So much so, that I now run them on my dedicated Rimfire ARs.
The 15-22 comes with some options too. I would say life is too short not to get yourself the 15-22 and the handguards!

I went for the 15-22 instead of an upper, as you said the uppers cost almost as much as a 15-22.  You also wont have to mess with swapping the uppers around, which will probably lead you to building a lower for it and then you'll have another $150 in it.  Save yourself some time and money and get a complete rifle.

15-22 as the bolt release is operational. Yeah it's a light gun, but correct operations to build muscle memory is paramount

Dedicated upper, better mag adapter, SW MP 15-22 Mags.

I have both. I never use my dedicated upper anymore. Yes the 15-22 is all nylon, but it just works. It's a blast to shoot. Mine eats everything including Remington ammo. Besides if you catch one on sale at Palmetto State Armory you can score one for $299.00. Mine was $317.00 shipped to me. A lot cheaper then a dedicated upper.