945 or 935?

I went from a 935 to a 945, and frankly, I didn't find the upgrade very worthwhile.  It's less than a $200 price difference between the two (Amazon), but it just depends how up to date you want to be. 

Tricky question. I totally agree with rtperna, most of new features aren't worth that money but... If you're using FR935 for cycling you will miss out maps and climbpro. You can still use the Edge 530 for cycling but then FR935 doesn't have training load focus... Bottom line if money isn't an issue I would get the 945. You never had the FR935 so you won't find yourself wondering whether the upgrade was worthy or not.

The big advantages of the 945 over the 935 are maps and battery life. Music and GarminPay are also new features that may allow you to ditch your phone and wallet while running.

If you are an ultrarunner, these features may well be compelling.

If you are a triathlete, then not so much - there isn't that much that the 945 does over the 935 that is essential, and it largely becomes whether the "nice to have" features are worth the extra price.  (The 945 does offer some slightly more advanced training metrics over the 935 - including Load Focus, and Heat and Altitude Acclimation.)

Physio True-Up allows you to intergrate most of the data on to your watch, even if it was recorded on your Edge - there is no real advantage to gathering all the on a single device. If you just run and ride as single sports, rather than as a multisport, then the 645M may be a newer generation watch that meets your needs at a lower price point.

Do you want better GPS accuracy? => 935
Do you need any of the new metrics/stuff that 945 have? => 945
Do you need maps? => 945
Otherwise 935 is enough.

Own plus/minus list:

945:
+ Maps
+ More routes can be on the watch
+ Some of the new Firstbeat stuff, data is always nice.
+ Faster CPU (If you have starred many segments, it can handle them better)
- GPS

935:
+ GPS