This post is about devices and carrier support, not OS versions. Quick thought/response about OS though...
My Nexus 7 (2013 LTE ver) is running 4.3.1 and it is still a great device. Google Play Services is up to date and depending on how you look at it that is perhaps more important than having the absolute latest version of Android. Further details here http://arstechnica.com/gadgets/2013/09/balky-carriers-and-slow-oems-step-aside-google-is-defragging-android/
Now back to the devices...
If you are thinking of getting either of these devices then using the +T-Mobile
service is a great option. I've been surprised how good and widely available their LTE network is.
I started with my Nexus 7 on T-Mobile and was able to also use it as a mobile hotspot, I subsequently switched it onto AT&T to check their LTE availability in my surroundings and as soon as that AT&T SIM went into the device the settings menu for activating the mobile hotspot disappeared even though my plan allows tethering. I spent a long time on the phone to AT&T and in the end they just told me that because they do not sell the device themselves they do not support tethering on it even though the AT&T plan I have allows tethering. There are posts on XDA about a mod you can do but you have to decide if you want to go down that path. Perhaps a carrier like T-Mobile that doesn't still shackle an unlocked device is a better choice?
My Nexus 5 has been on T-Mobile since it arrived and I do not see any reason to change that. As I sit here now writing this message I have 3 bars of T-Mobile LTE service and 1 bar of AT&T LTE service.
Both the Nexus 5 and Nexus 7 are fantastic devices at very good price points. T-Mobile also have well priced plans with no commitments.