Until just over a year ago I had an iMac with a 640GB internal drive and I likewise had to move my library to an external drive, in my case USB 2.  Its a perfectly viable option.  And if you can afford a Thunderbolt drive it should be very fast and responsive as well.  Even USB 3 is worth considering for this.

Are you using a managed Aperture library? That might explain why it is taking up a lot of space. 

If so, another solution on your new Macbook Pro would be to convert to a referenced library– put the masters on the external Thunderbolt/USB3 drive, and keep the Aperture library on the internal SSD drive. Also, there’s other advantages to that approach besides performance. But when using this scheme, don’t forget to keep a good backup of your masters and the library on another external drive.

Some weeks ago I read a speed test of Thunderbolt versus USB3. Conclusion was that the latter is a bit slower in real life (copy amulti gigabit file). The advantage of USB3 is its price, so my advice would to buy that one.

I’ve worked referenced for a few years and let my library run wild on LaCie Thunderbolt drives. The speed difference while editing with photo’s on my internal 750GB SSD v’s my external drives is pretty much non-existentant these days. I even used to run Aperture and PS CS5 from my external HD’s but I don’t do that anymore. I don’t have any USB3 drives but I have 3 thunderbolt drives daisy chained and the 6TB LittleBig Disc’s ROCK and it’s very, very fast. USB3 must be excellent then and it’s SO much cheaper. The newer thunderbolt drives are also much quieter than the original 2TLittleBig which is very noisy.