upgrading old macbook pro vs buying new one

Here's the deal: I have a mid 2009 model MBP, dual core 2.66 ghz, and 4gb RAM. Not the base but a little upgraded, and it's treated me very well. However, it's slowing down with every OSX update, and haven't even upgraded to mountain lion yet, still on lion. 320 GB hdd that I'm constantly filling up.

The Dilemma: Should I upgrade my current model with an SSD and more RAM? Or just buy a new model for the retina display?

I know these are all preferential things and you guys can't make that decision for me, although if anyone on here has upgraded there old model and the performance has decently increased, I'd appreciate it if you chime in.

A note: the processor would have to stay the same obviously, and the dual core has seemed to work fine for me, I don't do video editing all the time, maybe once or twice a year, and the retina display isn't a deal breaker for me.

$$$$: Buying a new MBP Retina is about $2800 for 2.6ghz quad core, 512gb ssd model. Upgrading mine would be about $450 for a 512gb ssd and about $100 for good RAM.