I agree with maflynn above.
It's not worth it to put much money into a 2011 MBP.
Having said that, it -might be- worth putting a -little- money into it while you're waiting for the 2018 MacBook Pros.
My advice:
Buy the CHEAPEST SSD YOU CAN FIND (not large capacity, 250gb at the most).
Put that into the 2011.
It will give you a very nice speed boost for the time being.
Leave the memory alone. If you feel that you must "bump up" the RAM, just buy ONE 8gb DIMM, and change it out with the 2gb DIMM in the "uppermost slot" when the back is off. That will give you 10gb, all you should need until a new MBP finds its way into your life.
When the time comes for you to buy a new MBP, you can then take the SSD back OUT OF the old 2011, and re-purpose it to be an additional drive at your disposal. Or -- if the 2011 is still doing ok, just keep the whole computer "as your backup".
I WOULD NOT recommend a 2017 MacBook Pro.
Too many reports of failed keyboards.
MAYBE they'll fix this in 2018 -- no way to know as of yet.
I would agree about getting the cheapest SSD you can find... caveat is get a new'ish model, i.e not much more than 1-2 years old... SSD's have gotten faster and cheaper. How much capacity will purely depend on how much space you need

If your current 320 is pretty full, I'd say it's not worth the sweat to squeeze down to 250; the $ diff between 250 and 500 is only like $30 these days.
Also, negotiated speed of 3 or 6 really doesn't translate into any noticeable difference. My early 2011 came with one that ran at 3, and I replaced it with one that ran at the full 6, and I didn't notice any difference at all. That being said, pretty much close to all, if not all, the newer SSD's will run at 6.
If you do get a 3rd party one, just remember to enable trim, which you'll have to do manually.
The cheapest (non off brand) ones I've found are the Sandisks, and I'm put one into an iMac, and one into a Mac Pro, and they're fine... pretty much transformed the machine.
And, I'd definitely go with at least 8Gb of RAM... Fishrrman's rec of 1x8 Gb chip to get you to 10 is definitely the best way to go. Personally, I like to get the ones that say "for Mac". Mac's seem to be more finicky with RAM than PC's. While I've gotten non "Mac certified" to run fine, I've also had some not work. Just worth the hassle of returns for me... If you want to save a few $$, you can find lots of them on eBay, many new.
I use my early 2011 as a spare, and once in a while when I have to pull it out, I'm pretty impressed with how well a 7 year old machine still runs.