No, because the MBP (even the 13" model) and the MBA target different types of users.
The MBA will almost certainly use
ULV (ultra low voltage) Sandy Bridge processors, because the MBA has a smaller battery and is less able to evacuate hot air efficiently. In other words, it'll be using the same basic architecture as the processor in the 13" MBP, but it'll be clocked much lower. Ergo, significantly less raw performance. Probably not noticeable to most end users (particularly in light of the default flash storage), but very much an issue for developers like me. Running compiles on a ULV mobile processor would cost me a lot of time. As is, I try to use my desktop for the heavy lifting, but I often have to rely upon my secondary computer (13" MBP) to do it--and having the full power processor is well worth it.
Better in what way? The "integrated flash memory" in the MBA provides no performance advantage over the sort of "external" SSD you'd install in a MBP, because they both use the same system bus. Planting the chips directly on the logic board is just a physical change made to facilitate the narrow form factor (and, if you're a bit of a conspiracist, to increase the end user's dependence upon Apple for service/repairs).
Likely, but not confirmed. Even so, this technology should be considered a sort of vaporware until reasonably priced consumer-grade devices hit the market. Until then, the idea that you can supplement the MBA's limited internal storage with a Thunderbolt-equipped external drive is meaningless. Besides, external hard drives are heavy and bulky--it's actually more ponderous than having a drive inside the machine itself. If you end up carrying it with you anyhow, doesn't that defeat the purpose?
There's always a compromise between portability and power--simple physics.
Several others have already covered this and I support their points.
Another relevant point: with the move to SB processors, it's virtually guaranteed that we'd see the MBA adapt the same HD3000 GPU that the 13" MBP uses. The HD3000 is integrated onto the processor die and performance scales with processor clock speed. This means in turn that the MBA will probably have slower graphics performance than the 13" MBP, a characteristic which will be further exaggerated by the fact that the MBA has a higher screen resolution (more taxing on the GPU).
To keep it apples-to-apples (bad pun intended
), let's assume that we equip a base 13" MBP with a 128 GB SSD and pit it against a 4 GB equipped 13" MBA. Same price (~$1400), but the MBP has a much faster processor, more ports, marginally better graphics performance, and more upgrade potential (RAM/HDD). So again, you're looking at a compromise between power and portability.
As for the 15" MBP? Again--power and portability. And also price.
If my notebook were only my machine, I'd absolutely spring for the high end 15" model. But I like the power of a desktop, and so my notebook is a secondary device--it's my road warrior. It sees enough important use that I'm not willing to take the performance hit that comes with the MBA, but I'm also not going to spend several hundred dollars more on a 15" MBP that's going to sit in the closet whenever I'm at home. The 13" model is a perfect compromise for my needs and wants.