I just made a similar decision and went with a Seek Compact Pro.
It offers much higher resolution and 15+ frames per second(if you have a PC or a powerful phone), over the FLIR One's 80x60(third gen is 80x60, only the $500 pro version is 160x120), and MSX doesn't give you much advantage at that resolution difference - plus you can focus the Seek Compact Pro without any hardware mods, and it works at around 15cm.
There's also software to control the Seek cameras on Windows and Linux(though no temp readout yet).
If you'd like a professional tool, the FLIR E4 hacked to E8 and a focusing aid tool is a better option. It provides better images than any of the phone cameras, with only the Therm-App($1000) coming close.