What sayeth thee o' Tone Lords of The Ether?

It's two different animals. Increasing RAM helps your processor and can significally speed up your MacBook, if you're using RAM-hungry apps (e.g. image editing, video editing etc.). Using an external hd will give you more storage for files.
What you need more only you can say.
If the choice is a bigger internal or an external drive I'd go for an internal, as long as you don't need the speed that an internal flash drive can give. If that is the case, I'd chose an internal flash drive just for system and apps and an external for files.
Or, if you can, swap the optical drive for a second hd and use an external optical drive (which is in most cases used much less than storage space).

I need more space for storing sessions, charts, everything.. AND I need mo' speed.. hot dirty sexy nasty speed..

Makus is right but here's what you want:

1st Get the largest/fastest internal SSD DRIVE you can afford, for your OS and PT.(Up to 100,000 IOPS 4k Random Read Up to 95,000 IOPS 4k Random Write the higher thease #'s are the faster it is)

2nd Max out your ram (put your Mac Book Pro model # in at Everymac and it will tell you what type ram and what it can be maxed out to, however my Mac Book Pro: said 6gb ram and I successfully run 8gb ram and a TerraByte Hard drive) this will give it a shot in the but!!!

3rd get the largest external drive (this is a USB if you have a Thunderbolt or Firewire port you may want to get a drive that has it) you can afford and use it for your data drive ei: all pt sessions, charts and everything.

The 2 drive set up is what PT suggests that you use. Keep in mind these are just suggestions of devices and you can spend much more or less according to your wallet. post your model and I will help you get started.