Upgradable MacBook or a 2011 MacBook Air?

So, I'm in the market for a new laptop. I made the switch to MAC with my desktop about a year ago. I have a iMac operating on snow leapord running applications such as adobe creative suite master collection 5, office for mac, parellels, windows 7, quickbooks 2012 (Win7), and MS Office Professional (Win7). I need a portable version of my iMAC for traveling. I would like the ability to do basic graphic design work while traveling or sitting in a coffee shop but I would leave the intensive stuff for my iMAC. So I do intend to load my adobe creative suite onto this new laptop.

My question is this. If I want a portable version of my iMac, should I go with the latest macbook air or a used macbook? Please consider that I want this laptop to last at least 5 years. If I did the used macbook, I plan to upgrade the ram to 8MB and the harddrive to 750GB or 1TB. I would find a used mid that would allow me to do this. I would also take the optical drive out and replace it with a OptiBay hard drive with the intel X25-M 160 SSD drive. My applications would boot from the SSD drive. If I did the 2011 Macbook, I would get the one with 1.5Ghz Duo Core Intel-Core i5 with 4GB ram and the 128 SSD harddrive.

When I present my thoughts to the local apple store repair guys, they tell me to go with the 2011 macbook. But they are also salemen. I love the weightless-ness feeling of the macbook air but will it last for what I need it to do for the next three to five years? Would it be best to go the use macbook option and just max out ram and the hard drive? I don't intend to upgrade any of my application any time soon. So what would you do?