Probably neither. It goes on sale periodically.
The solar version of the x40 series notches the price up US$100 from the baseline. If someone really wants the solar with no compromises, the 1040 solar sells for about $600. One thing I learned while reading a bunch of reviews is that the solar charger only works when the Garmin is turned on, so it isn't like you can leave the thing on a picnic table for a few hours.
I have a 530 now that my kid wants (back), if I upgrade. I am almost tempted by this, but there are a few things I will miss:
(a) wifi (a convenience at the end of a ride; I don't have to use a phone to sync it)
(b) No electronic shifting integration -- I only started using this a few weeks ago, for a new mountain bike, but I will miss it.
(c) size. I am one of the few who prefers the smaller profile of the 530/540 etc.
The price, however, continues to tempt me. I would buy it from REI to cover myself for potential buyer's remorse, but they no longer list it.
The navigation is apparently significantly improved from that on the 530, which is a major source of frustration.
The natural choice for me is the 540 or 840. Apparently, unlike the 530, the 540 is a bit of a pain without touch-screen, but $450 for the 540 vs $250 for this, with a few compromises, is a big price jump.