Would the Pro 3 and Take 5 work well together for ambient/atmospheric music? (Here's a sample of the tracks I've produced: http://soundcloud.com/douglas-1)
These would be my first Sequential synths, so I'd be interested in any advice.
I can definitely recommend the Pro 3. It's probably the best mono synth on the market. It does just about everything one would want to do with a mono synth, and it's almost as versatile as a eurorack in a box. It does all sorts of stuff which would work great for ambient, along with almost any other genre. If you don't mind recording one part at a time and stacking them, a Pro 3 may be all you need... it's easy to build entire songs with.
The Take 5 is a nice synth too, but it's designed to be simple and relatively inexpensive, rather than prioritizing power and versatility. It's not built for doing complex ambient tones... it's built for more traditional sounds -- for musicians who have acoustic instruments and want to add their first synth.
If you want a poly synth with deep sound design which does really crazy stuff, ambient tones where you can press a few keys and then just hold them (and maybe turn knobs) while the tone evolves for a long time, you might want to consider a Hydrasynth. Or a Peak/Summit is great too, with a juicier core sound but less convenient mod routing.
On another note, it's usually a good idea to only get one new instrument at a time... and then dive deep and learn it really thoroughly before considering the next purchase. Getting two at once is often a recipe for feeling unsatisfied with one or both.