Another options are: iMac (21", 27") MBP 17" MBA 13"
Any thoughts?
I'm basing my recommendations on using a 13" Macbook for the last 3 1/2 years and a 17" Macbook Pro (from work) for the last couple of months. Also, you might want to hold off a little longer as the Macbook Pro line is due for an update and it might be worth it to see what Apple does to them (http://buyersguide.macrumors.com/).
13" Macbook Air
-memory capped at 4GB ram -1440 display resolution -no disc drive
The resolution makes this great choice for a portable 13". But, you don't have a disc drive. You're limited to 4GB RAM and 256GB disk space. If you plan to run any virtual machines or dual boot into Windows - I'd pass on the Air. If you plan to do any work with iMovie and iDVD, that also rules this out with no disk drive (you can buy an external drive). Otherwise, it's a solid choice or portability.
------------------------------------------ The Macbook Pro line is due for a refresh.
13" Macbook Pro
-1280 dispaly resolution
This display size is a little small. That being said, I'm curious to see it Apple will upgrade the display when this model is refreshed. I would loved to see this form factor include the 1440 display that is in the MBAir. If you plan to spend a great deal of time working from the same desk and have a decent external display. This with a 27" Apple display would be a killer combo and I'd prefer it over a 17" Macbook Pro. But, if you're on the go, I'd pass.
15" Macbook Pro
-1440 display resolution (upgradeable to 1680)
My next personal MBP would probably be this one with the upgraded display. It's not too big and you've got a decent amount of screen real estate.
17" Macbook Pro
-1920 display resolution
This one is a little pricey, but at times the resolution is nice when using something like BetterSnapTool or Double Pane to resize windows on the fly to fill half a screen.
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CONCLUSION
The best all around value is the 15" Pro (with or without the upgraded display). I would only buy another 13" if I knew I was going to be stationary (most of the time) and would lean heavily on an external display.
There are two major differences between the 15" Pro and the 13" Pro: monitor options and computing power. The 15" is obviously more powerful, but you can also add a higher resolution, anti-glare, or both to your screen. These are non-trivial costs ($150-200 each, I think).
The 13" Air is the ultimate in portability, and people love their performance so far. I ultimately went with the MBP, though, b/c I use my laptops for years and questioned the Air's non-expandable RAM (4GB max).
On the 13" i'd go with an Air because where it loses in some places it makes up in the advantages, such as weight and portability.
These are the choices i'd look at when looking for another Mac. My recommendation for an all rounder is the 15" pro. It's not too heavy, thick or hard to work from. I know a few 17" owners and they have trouble opening it up and using it in a lot of places.
A lot of people swear by the MBA - I'm not convinced. Do they weigh less than my MBP? Yes. But can I swing the extra ~3LBS of weight? Yes. MBP has expandable ram, more screen real estate, and a disk drive -- I need Windows for a few applications I run and occasionally for testing - I don't even know if it would be possible to install it without the disk drive.
If not, get the 3rd 15" MBP and upgrade the processor, screen to hi-res and HD to SSD.
I just bought both recently.