Which machine is going to be your main, goto machine?
I think it depends on the circumstances. For day to day use when I'm not running real-time rendering demonstrations, I'll likely use the MBA. On those occasions when I need to do real-time renders while at a client location, I'll pick up my 15" MBP.
Should note that the 15" MBP, even with an i7 and 8 gigs of RAM, isn't the best choice for renders. I've full desktop machines that are far faster. But I do "mini-renders" with my clients on-site so its passable. I'll try a couple today on my MBA on my trip over to Vegas and time them. It may suffice, but haven't tried that yet.
I am LOVING the feeling of carrying this little thing though. The last time I tried to do a trip with just my iPad and the keyboard I was sort of stressed out about the limitations of the thing. I don't have that stress now....this MBA can do likely almost anything my MPB can, albeit slower or with less grace.s