Honestly that should be a pretty easy decision. I would go with the Tmobile G2X or the Nexus S. You would have to unlock the G2X in order to use it in Europe but that's not to hard to do, and the Nexus S comes factory unlocked. Both phones use the most common GSM frequencies so they will work around the world.
Both phones have easily unlockable bootloaders, the Nexus S has a pretty large dev community which keeps growing and pretty soon the G2X should be pretty large too. The G2X supports 4G (HSPA+) up to 21 mbs. The Nexus S is not a 4G device but like any other android phone you can take advantage of the 4G speeds up to 7.2 mbs down which is more than enough to stream anything without having to deal with a long buffering time.
Both devices can run Skype, so you shouldn't have a problem there.
I would also suggest the HTC Sensation 4G although i believe the bootloader is encrypted. But from reports they may release an update to unlock the bootloader. The Sensation will work in Europe just like the others you would just need to unlock it.
I'll list some pros and cons for each device.
Nexus S
Pros:
Stock google device (You will always have the latest software, which may not be important since you are a root user.)
Very quick and responsive
Looks awesome (When the screen is off it looks like there aren't any buttons.)
SAMOLED screen (The colors are beautiful.)
Awesome battery life
Cons:
Some users still experience random reboots, but for most that problem is completely fixed.
Plastic (It's made from high quality plastic, which in my opinion is really nice but for some people they really don't like the build quality
G2X
Pros:
Tegra 2 processor (Awesome if you want to play some HD games.)
Great build quality (has a really nice feel and weight in the hand.)
Stock device but i believe it's currently running Android 2.2 with an update to 2.3 coming soon.
Cons:
Battery life (most 4G phones have terrible battery life.)
IPS Display (It's not all that bad, but it doesn't look that great and some users experience light bleed through.
Reboots and other issues. (If you look in the G2X subforums there are quite a few users that have experienced reboots often, and other issues. But some users haven't had any of these problems.)
I can't list the pros and cons for the HTC Sensation because i haven't had a chance to mess around with it, but so far reports are saying it's an awesome device.
Also, welcome to the Android Forums. Hopefully this helps, and if you are looking for a phone with a hardware keyboard i would look into the Mytouch 4G slide. It isn't released yet but should be coming out soon enough.
And i don't know who told you that the G2 will not work in Europe with data. Because it will, it uses the same GSM frequencies as the rest of the Tmobile phones. Honestly i probably wouldn't consider the G2 unless you are looking to buy the device for really cheap and don't want the latest hardware and software. Don't get me wrong the G2 is great but it's been out for around 9 months now and there are a lot better options.