The Technics may be the one-and-done buy for the rest of your life. Then again, maybe the Rega could be as well.
One can certainly buy the Rega Planar 3 and apply no upgrades. Over time, I've come to think of that as perhaps the more sensible way.
Fairly soon, anyone new to Rega will start hearing about upgrades, both Rega and third party. There are many available.
From Rega itself: an external power supply is the main one
From 3rd party:
Acrylic or Delrin platter
Machined subplatter
Replacement counterweight
Replacement feet
Record weight
Machined pulley
Double pulley for 2-belt configuration
If not exactly upgrades, there are also some accessories from Rega to allow arm height adjustment.
As some of the Rega original parts can look a bit cheap (exhibit A: the feet), some of the 3rd party options might be more cosmetic rather than sonic improvements, though you can be sure they'll tell you it's sonic. The thing is if you apply some of the less-minor upgrades, say a power supply (about $400 in the US), a new subplatter, platter, and counterweight (total $750 for Groovetracer, rather a lot more if you opt for their premium versions), quite a lot of cost has been added to the turntable's original price.
With the SL1500C there's no need, not really any way in fact, to rebuild the deck like that, or add functions, and most people probably won't upgrade anything at all (I'm not counting the cartridge, which is something you can do for any turntable).
One thing I didn't mention, but maybe someone else did in this thread, is that along with swappable headshell capability, the Technics has adjustable arm height. With the Rega, that's one of their "you don't need that" or "all you have to do is" items [followed by PITA procedure]. As Rega likes to say, "we have omitted all the usual gimmicks".
If OP does decide on the Rega, it might be a good idea, if they're likely to want the power supply later, to just spend the money now on the Planar 3 RS, though it's about $1700.
That bundles the Neo PSU mark 2 and some other improvements over the basic Planar 3 model. Apparently.
I see more value in what the Technics SL1500C gives without any additional thought or work, whether comparing it to the RS edition or the plain old Planar 3.