Hi Meezo,
I use a MacBook Pro for SketchUp. I would have some doubts about the performance of the Mac Air. It probably will do for what you need. I suggest that you ask the Apple dealer to download SketchUp at the store and load one of your SketchUp files. Then you can be sure it it works for you.
Money is tight these days .... tell me about it π I have purchased my last two MacBook Pros from the Apple Refurb Store. The computers are as good as new and have been discounted up to 15%. I see you are located in the USA, so check out this link, http://store.apple.com/us/browse/home/specialdeals/mac It should be possible to get a good spec refurb MBP for the cost of a new Air.
Here are a couple of screen grabs I did!
13.3 MBP.png
15.png
I'd say, go for the 15".
Mike