I don't know actually. Maybe when it's on a discount.
See the differences, unless you want to play Japan or China it seems like minor tweaks. It's worth keeping in mind that a lot of costs and benefits are different between HTTT and DW though.
Funny, one of the changes concerns Personal Unions. Getting a PU isn't that difficult, but getting something out of it is. I got into a PU with three province Brabant, and because I was bored I integrated it (which is a feature of DW) into the evil french empire. Now I read that I should've waited until I inherited that lousy part of Europe, because then I'd have gotten cores on those provinces. Now all I have are rebels (good for army tradition).