If you have no status with Carlson, I doubt you will get the upgrade. I have been offered upgrades to Business Class rooms with points at check-in versus paying cash, so it can be done. Surely location is a factor too? The other two brands you mention are in differing parts of town. 50k points can be very useful if used judiciously but it's a value judgement.
From the place we need to be, I reckon all three are equidistant (maybe the CP is another 200m further away, but no big deal).
The problem I have is I risk paying £50 more for a sub-standard room, that in turn gives me a slug of points that I'll end up having to use on another substandard room.
So for £150 I get two substandard rooms, or for £100 I get one good room (at the CP).
Alternatively I pay an extra £25 to get a better room at the Radisson to start with. This makes the total spend £175 and I'd have >75k points as a result, giving me enough for 1 redemption into a business class room.
Perhaps that's how I need to address the matter....
Thanks all