Other than the extra gear is there a reason to get the 11 speed Ultegra over the 10. I’ve been looking to switch to DI2 and some of the things I’ve read say you can’t upgrade the 10 speed software and it’s not compatible any of the 11 speed products. Not sure if this is true or not.
Don’t want to go with the 10 speed then be stuck if something breaks. Already checked and my wheels can handle 11 speed so would there be any reason not to go with 11 speed?
I hear 10-spd Di2 derailleurs are already hard to find and going for quite a bit on the used market. The only reason you would stick with 10-spd would be if you already had a significant investment in it and even then it probably wouldn’t be worth it in the long run.
Most of the Ultegra 10 speed and 11 speed components are cross compatible. The only software constraint on mixing is that both front and rear derailleurs have to be either 10 or 11 speed (this didn’t used to be true, but became so with a recent firmware update). So the shifters, wires, junction boxes, battery, etc, can be used in either a 10 speed or 11 speed setup, but you can’t run an 11 speed rear with the original 10 speed front.
The same is not true of 10 speed and 11 speed Dura Ace, which are completely incompatible.
Your upgrade path from 10 to 11 speed Di2 is very cloudy based on a 2014 firmware update. This video linked below explains this. I have 10 speed and am happy with it but I bought a used bike from a bike shop manager and saved myself about $1500 so that eased the pain of missing a gear 
If you are not getting a big discount, go 11.
http://velonews.competitor.com/2014/11/video/ask-a-mechanic-shimano-di2-11-speed-upgrade_352301
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