G'day Luke,

There's no real way to compare these I'm sorry, as the warmth of a liner cannot be independently tested or verified its literally a case of best guess.
I know the Thermolite Reactor Extreme Liner has a 15ºc claim, but in my personal experience with owning this one, you'd be lucky to get a few degrees of extra warmth.
It's one of those situations where your physiology and entire sleep system has more impact on the effectiveness of a liner than the liner itself.  I'd lean more towards a merino liner, as I find merino to be a better material for temperature regulation than synthetics, but if you are finding you're consistently sleeping cold, there are some other things you can do to try and offset that aswell.

No worries!  I'd lean more toward the XTM in that case.  I used the Thermolite Reactor with a 0ºc sleeping bag, on 3ºc nights overseas, and I froze my butt off.
Granted my sleeping mat didn't have a super high R-value, but long story short, the liner did very little to help.  Try the XTM if you haven't already made your purchase.