I used to use a MBP 2010 13" for all my tasks like Photoshop,Word,small gaming because It can't handle much and it had a 256gb Sandisk SSD. I know it's bit different from your 2011 CPU wise but depending on the model the 320m is similar.
Anyway! I got a MacBook Air 2015 today(2016 is a 2015 I believe?), I was a little skeptical how it would perform being stressed by my multitasking, using FaceTime, Games like Left 4 Dead 2 & Garry's Mod(Even Call Of Duty Black Ops works on this!). Turns out I was skeptical for nothing and it impressed me a lot.
CPU - This crushes my MBP 2010, the i5 is really fast. It handles every task I throw at it with ease. FaceTime used to be one of my biggest issues with my MBP 2010, it would heat it up and make the fans turn on constantly. On the MBA? Not even a problem, it's such a tiny task. If I remember correctly, your 2011 should use a first generation Core processor. Even then, the performance increase should be significant(correct me if I am wrong)
RAM - I got the 8gb version, stay away from the 4gb version. Ram can't be upgraded and will limit you in Sierra(not too much, but just avoid it) or future macOSes that require more. My 2010 had 6gb DDR3 and was fine but that was upgradable so I didn't worry about it.
GPU - This kills the 320m, I found OS X gaming to actually be playable!

Intel HD 3000 in the 2011 should be similar to 320m. Besides gaming, I can use Photoshop on this with no problem. Youtube playback is smooth. The 1440x900 Resolution and screen is fantastic too
Battery - This is a major one! The battery increase for me is great. I use my MacBook Pro at school and find myself at 20-40% usually. I can't bring a charger as it's too much weight for me(hurts my back) and causes clutter. So I just disconnected my MBA from the charger, I have been multitasking for around 25-30 minutes and I have 100% still, this would've taken 10-15% away if I was on my MacBook Pro 2010. I've also been using it for FaceTime and after around an hour, I've lost 10%. The 2011 MBP actually lost battery life in the Core i processor transition so it should be even better to you.
Heat - With Macs, heat is always a problem especially on a older model. The 2010 was no exception(the Penryn processor helps though), I found myself with the temps at 50-80c when doing intensive tasks. With the MBA, temperatures sit at around 30-40c when doing what I'd normally do on the MBP. Gaming got the temps to 70c-80c(fans kicked in as well) but being this is a MBA, I wouldn't game much. Every once and a while maybe. I don't know your exact uses but if your 2011 can handle it fine, the 2015 will do it more efficient and cooler.
Storage - Probably my only gripe would be the 128gb but a external SSD/HDD can solve this problem. Luckily Apple didn't solder the SSD but used a proprietary connector. You can also upgrade to 256gb and 500gb but you'll be spending more. This also has Thunderbolt 2 so you can connect a HDD(however the USB 3 port could handle that, I would see a SSD taking advantage of this though.). As I just mentioned, USB 3 so devices that can handle that will be quick.
Also the iSight camera quality(720p) is great, there are also a few more software features as well. PowerNap is a nice addition, which allows your Mac to check mail, updates,etc(if turned on in preferences).
I would go for the MacBook Air, spending $250 seems like a great cost for all the improvements. Good luck!
