935 or 735xt

935 because of better battery life and more accurate resting heart rate.

I use a forerunner 935 for the past 4 years

OHR doesn't work for me, so I use a chest strap. 735/935 would't make a difference for me.

They have the same basic navigation (follow the breadcrumb) which I use a lot. 

They have about the same firstbeat functions, like training load, vo2 max etc. But I don't really use that (nor trust it very much)

735 is smaller than 935. I selected my 935 because it's one of the smaller sportwatches. Suunto 9 was a alternative but it is huge. So are Garmin fenix options). Some people find it harder to read the smaller digits on a smaller screen. 

Batterylife is a difference. Not because I do 20+ hour ultras. But because I don't have to charge my watch every other day and usually it has enough power (15% or more) left to do a decent workout. Also batterylife will become less and less. My 935 started with 24 hours on gps. After 4 years only 10 -15 hours are left. So if a watch starts out with 10 - 15 hours batterylife, after a few years half is left and then it starts to annoy you. Last, batterydegeneration is related to charge cycles. A watch that you have to charge once a week has way less chargecycles than one you have to charge every 3 days. So a big battery will not only last longer in hours of batterylife but also in years you can use your watch.

So, in the end it comes down to 3 thing

Price: how much money are you willing to spend? Moneywise, the 735 is probably the better deal.

Size: how "small" do you want your device to be. For me, my 935 is the max. For others 735 is to small. 

How long do you plan to use your device? The one with the bigger battery will serve you the longest.

Sure, a 935 has more features than a 735 and newer devices have even more features. But most are just gimmicks you don't really need.

I'm happy to use my 935 for an other 2 years (at least), but was probably just as happy with a 735 (until the charging starts to annoy.