The details are covered in the numerous reviews online. Have you read them?
In essence: The NEX system uses DSLR sensors in a PnS style body. You can get DSLR-quality images in a compact package.
- Good image quality
- short flange distance is perfect for using old rangefinder lenses, like Leica/Voigtlander lenses, etc.
- Small enough that Sistic goons don't mind it at concerts.
- Slow shot to shot performance, like a PnS
- lack of buttons and knobs for manual controls
- Menu is a PnS menu, it's designed for n00bs and non-photography people.
- Small array of lenses. More coming, but it's a slow trickle.
Essentially it's a really great backup/travel cam, but I would not replace my DSLR with it.
What is your photography style. How do you like to shoot? what type of photography. Plus do you have large lenses collection? And have you handle the NEX yet? It is really difficult for anyone of us to give you any advices. Because it would ultimately boils down to personal preferences.
Just know that IQ will be similar, D5000 should have more manual control (but if you are the type who shoot in auto mode, then it will not matter much), less or no shutter lag, etc.
One more thing, look to ergonomic of the cameras. For me, DSLR feels better, I have a 7D and a EPL1. 7D had better grip (my own opinion) and it feels good in my hand... but it is freaking heavy. EPL1 is light and easy to carry around, but I don't actually like the grip too much.
In the first place, you created your own headache. In the first place, what are your intentions of changing to NEX-5? If you can justify the reasons, then change it. If you can't, then stick to D5000 and enjoy the photography. Personally, I don't consider D5000 a big camera and don't see the need for you to change system to NEX-5 just for travels. Not to mention that it's a gift from your gf that you shall learn to treasure.
Give yourself more time to explore the D5000. Maybe you will have a better understanding of the various camera systems and you will have a clearer picture of what you wanna change to?
Since you already have one, why not use it ... eventually you'll find the max of its capability, related to your day-to-day use ...
By then, you'll get the sense of whether you want/need a smaller/lighter cam, better lens, bigger sensor, etc ... and you can look at NEX or whatever camera that fits the bill.
For example, after a while, you get lazy to bring the camera out because it's too heavy or too bulky ... well maybe it's time to get a lighter/smaller one ...
IMO..as it is a gift from ur gf.. might as well treasure and learn to use it..
to get Nex5.. u need to top up the price diff.. unless the nex also a used cam.
anyway, i also think that D5000 is better than Nex5..
as a man.. i think it will be no prob to lug another 900g cam for ur travel..
If you are just looking for a camera... which erm produce better picture quality and more versatile than compacts. NEX is for u..
The size isnt big at all when u put it next to a DSLR. i would say.. NEX is a perfect tools for your everyday n travel camera..especially if u love to take pic everywhere n anywhere..NEX is light n small enough..
To be honest if it's a gift from a beloved one i wouldn't think of selling it away as the sentimental value is there. To other people it might just be a 2nd hand d5k but to me it's something which my love one bothered to go down to the shop and buy to give it to me.
If you give your mother a present/gift and she sold it away to get some cash, how would you feel? Please think of others before yourself. Your GF spent like how much to buy you a DSLR and now you want to sell away for a PnS?
TS... my advise is still the same, go and try up the NEX-5 first to see if it suits you. Photography is not just about how good the IQ of the camera is because nowaday, all of them have decent output. One important factor is how you enjoy this hobby and it actually boils down alot on how you feel about the camera that you are using.
As to the D5000 is a gift from your girlfriend and whether she is okay with you selling it away or not is seriously none of our business and we are not experts in love matter or any of us actually know the character of you and your girlfriend, so if you think it is alright to sell your camera away to get another one, it is your own decision and whatever outcome it is your own responsibility to face it.
Anyway... go down to the shop, or borrow from a friend, whatever, try out the NEX-5 and see if it actually works for you and if you enjoy using it. Size is one thing, erogonomic (location of buttons, grips, etc), user interface, etc, plays a big part in you being comfortable with the camera.