Device memory in Galaxy S2 vs Nexus 5 My biggest issue with Galaxy S2 that I currently own is "device memory" being limited to 2 GB. I don't have any music and rarely take photos, so I end up with lots of free space in "USB storage" (~10 GB) and no free space in "device memory". Getting external SD card helped for a while, but I still have to remove some apps to install new ones and constantly clear app cache and do tricks with *#9900#.

Is situation any better with Nexus 5? I wasn't able to find any information on Nexus 5 "device memory" size and the fact that Nexus 5 doesn't have external SD card slot is worrying.
GSM Arena to the rescue! Note if you just go to the Nexus 5 site you can see the phone specs, and the models available.
The Nexus 5 has a 16 Gb and a 32 Gb model, with no extensible memory. You can expect to lose between 5 - 7 Gb on most modern devices out of the box, as this is partitioned to various tasks, mostly space for Android itself, and any future updates that may be larger size.
It's impossible to say how much you will use, but a 32 Gb model should do you fine for most apps, games and general use.
I have looked at at a Nexus 5 and the /data available to the user is:

26.7 GB for the 32 GB
12.55 GB for the 16 GB

It is theoretically possible to re-partition a device, however, you are far more than likely to brick the device completely, and I would not recommend it to anyone who is not developing custom firmware of their own. And even then with extreme caution.
There are a lot of partitions, but here are the main Nexus 5 ones:
/system 1009.3Mb
/cache 689.8Mb
/persist 15.8Mb
/firmware 64.0Mb

For full details see this XDA thread