The rickman frames have too much rake out of the box, makes them an excellent road bike and a fast track (read: long track) racer but not a good short track bike. Basically any place a Ducati bevel 750/900 SS would be good a rickman would be good as well. You still have to set them and that means mods that are beyond show room stock. I have a friend of a friend who races a rickman 750 Honda in AHRMA and WERA and his bike is competetive but he dropped the stock swing arm in favor of a stock one off a 75-76 750f. This opened up the wheelbase on the bike (he felt that the combination excessive rake and trail and short wheelbase was a little twitchy). Required no mods to the frame as the rickman pivot is wider than the stock 750, but it did require custom spacers and a longer pivot bolt, an you have to machine the spacers on a mill since the need to fit into the slot where the rickman chain adjuster is.
As far as stiffness is concerned it is a non issue. Both frames are plenty stiff for racing work.
As far as both frames go, the rickman has a higher pain in the ass factor. The rickman frame uses a lot of proprietary parts (basically everything but the engine, sidecovers, exhaust, and electricals) which are really rare. Tracking down an original Rickman 42mm front end is going to be as much fun as a punch in the groin, and no you can't use a stock 750 front end. I have heard that there is a 38mm Ducati Pantah front end that might work but have you seen the prices on duc front ends? Also, the rickman frame is braised and nickel plated construction ( not welded). The plating is structural to the joint as it acts as a hardener for the joint. That means you have to keep it shiny and if it is a race bike you have to inspect the neck for cracks every season. Road bikes aren't ridden hard enough for the neck cracking to be an issue unless you crash it.
Since seelys are really hard to track down I assume you are considering the repop seely frames being made. Since it is a new frame with modern construction and materials and can be ordered and setup any way you want it def has a less of a pain in the ass factor than the rickman. My friend Kenny Cummings races a seely norton commando with one of these new frames and the bike is an unstoppable force to be reckoned with in vintage racing. If you are talking about an original seely then good luck finding one - it's easier to find a pecker on a cow than a useable old seely frame.
