I wanted to offer a public response about a private message I received. We do NOT concentrate in any particular product type or space. Far from it. We have literally thousands of spread combinations in every worldwide electronic exchange. My feeling is that price trumps everything and that's what we base our analytics on. If it correlates appropriately, and if the exchange will offer us margin credits, we are definitely good to go. I personally could care less if it's DJ EuroStoxx vs FTSE or a CBOT 2's-5's-10's Butterfly. If it looks sexy and sets up well with good targeting - we take it.
If it's electronic we spread it. We have special technical studies and some techniques that are designed especially for spread trading.
Having said that, most of my clients certainly tend to spread futures because it is so inexpensive to do in terms of overnight margin. Futures meaning stock indices, short and long term interest rates, grains, softs, energy, metals, etc. etc.. A client can typically carry several different spreads overnight for just a few thousand dollars of margin - especially if you are staying away from the more volatile products. The sizing is usually quite good, and scalability is excellent in the major products. We like to swing trade, and the volatility and modeled trading range of a particular spread determines that specific trade's holding period and profit / stop-loss targets.
My clients seem to be very satisfied with the approach, and we have refined it quite a bit over time.