Some can live with 128GB of storage happily, others would never consider it. So enough—enough for what?
It depends in part on not only what your uses will be, but how you prefer to work. With Photoshop, for instance, RAW picture files tend to be rather large. If you only ever deal with a few, and a modest photo library, then you may need relatively little space. If exceeding that (and if not today, then in time), then how happy will you be transferring files from (what is likely to be) a slower HDD external, or just working from files on that slower drive? Generally, and of course this individually subjective, one would prefer to have all that commonly accessed on the internal drive. All the more if it is SDD, and the external not.
One exception would be where video is involved, as it could be near impossible to have a large enough internal drive to deal with all of it. Even if you never work with it, just one HD movie from iTunes can be in the neighborhood of 5GB alone. That adds up. Files such as that can almost by necessity end up on an external, and in that case no big deal as iTunes can access them automatically once moved.
You may wish to pass on the Air for graphic design, as, even with a higher resolution, its screen is not all that good. That of the MacBook Pro is decent and satisfactory, with better color gamut, for one, than the Air. The retina is obviously the best. Look at them all side by side in a store and you'll quickly see the difference.
Would these all be purchased new? If not, then aside from GHz and anything else, consider the GPU. If you are not dealing with video or gaming, then not as much an issue. Otherwise, make sure it is new enough to have the Intel HD 4000. Even the Intel HD 3000 is a big step down in performance. The 13" retina comes standard with the Intel HD 4000, but only the newer models of the MacBook Pro or Air.
Whether the retina display is worth its price premium only you can decide. One might rationalize its need due graphic design use, although a lot of fine work has been done with the cMBP and monitors of similar quality. In my estimation the 13" retina, even with Apple's recent price reduction, remains over priced and under powered. It lacks the quad core ability of its larger 15" sibling; also without dedicated graphics, so relying solely on the Intel HD 4000. Which by most accounts seems just enough to get the job done with all those many more pixels.
There are also reports of these retinas developing serious image retention problems, this often appearing only months after purchase. In short, one is rolling the dice to an extent with these expensive Version A models. Even as otherwise tempting.
All the more if you could buy one at a discount. There are none presently available from Apple, but only a couple weeks ago a refurbished 13" retina, with 8GB RAM/256GB of storage, could be had for $1,359. That comes close to equivalency in what a refurbished 13" cMBP would cost if one then added a SSD and more RAM themselves. That could be a harder decision, or perhaps one not at all if really wanting a retina in the first place.
One caution, though, if opting for a cMBP with an upgraded SSD. Apple only supports TRIM on its own SSD's, not any third party as one would install on their own. With the retina this is decided, as it comes with one and you cannot later change it. So the advantage of the cMBP in being able to easily swap out the HDD for any size SSD you like. Or, also the option of adding 16GB of RAM, versus the limit of 8GB with the 13" retina. But back to TRIM, being generally advisable. Some feel they can live without it. There are other workarounds to enable it with third party SSD's. But with any software update by Apple, you may have to reenable it. Not to mention the hassle of dealing with SSD firmware updates. Buy the internal SSD from Apple and either of these potential problems is a non issue. But it will cost you. And possibly in other ways down the road if you opt to handle this on your own.
So where and what? With the advent of Haswell this year—likely seen introduced in the retina models, Air, possibly cMBP, in Q3, or autumn of this year—one may well wish to wait until then, if able. Particularly with the 13" retina, Haswell should decidedly improve both its battery life and graphics performance. The Air would also benefit from this, and may as well see some significant changes to its overall design.
Thus it seems inadvisable to buy any Air model presently, unless happy with what will shortly be superseded by something more tempting and powerful.
The cMBP in some respects the only safe bet, as a known quantity. It will be discontinued, possibly with this next model cycle. Those wishing the option of upgrading RAM and the HDD on their own will prefer it. Others not, as it will surely be soon discontinued and so somewhat dated. Although also currently representing the most power possible for the money in an Apple notebook.
And the retina? The future. If one off to a not always solid start in its first iteration. A lovely screen (if remaining as such, or then dealing in having it fixed under warranty), but not really penciling out on a cost per performance measure. If a refurbished unit at the right price, then an all the more tempting and harder decision.
