Both are fine cars. Becks (and I have one) are built to a more affordable price and lack the power windows, more elaborate frame, more expensive interior trim, and a few other touches that IM builds into their cars. I consider both builders friends, and believe their products to be the best possible at their respective price points. The extra money for an IM does allow for more time to be taken in some areas such as curing the fiberglass, finishing the paint and upholstery, etc. Both builders will do their best to accommodate the buyer's wishes, but the IM price structure supports more high-end custom work. Figure out how much you can spend, and if your automotive fantasy requires that you spend all of it.
EDIT: If you can hold out until next May, come to the Speedsters Meet Spyders gathering at the Carlisle Import and Kit Nationals. Not only will a whole bunch of us be there, but Henry Reisner (IM) and Kevin and Carey Hines (Beck) will be there as well. Talk to them, compare products, drive cars, and make an informed decision.