If you're looking to do web design and development the MBA is not the laptop for you. You are better off going with an upgraded MB or a MBP. The MBA isn't a "production" machine.
Please stop regurgitate the same lack-of-power misconception.
Of course MBA won't be as fast as full sized laptop model, but keep in mind that MBA has the same processing power as PowerMac G5!
MacBook Air, 1.8ghz, SSD drive.
Geekbench 32bit, score 2132
No apps, running, although NOT after a fresh reboot.
As a point of reference, MacBook Air scored slightly higher than
PowerMac G5 (late 2005)
Dual-Core PowerPC G5 2.5ghz
which scored 2105
http://www.primatelabs.ca/blog/2007/...e-august-2007/
Here is a thread with more info
https://forums.macrumors.com/threads/432141/
Note that MBA with SSD drive has the benefit of fast random read / write, which is beneficial for web based tasks.
It'll depend on personal preference and what is your current computer. If you value extra mobility, then get MBA. Personally I have no problem moving around with my MBP 17" on a daily basis, but some people prefer extra light laptop.