Gear Combo Guru
Join Date: May 2007
Bikes: Spec' Tarmac (road), Ridley Fenix (commuter & fast tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder)
Differences between 5703 (new 105 triple) and 6604 (Ultegra SL triple) groupsets:
The large chainring on the 6604 has 52 teeth, the large ring on the 5703 has a 50-tooth big ring. Therefore, the 6604 will allow you to travel 4% faster (about half a gear on the rear) for a given cadence. The shape of the front derailleurs for each group also differ to allow for this difference.
The 6604 group is lighter than the 5703 group. I'm not sure by how much, I'd guess by about 200 grams total, maybe less.
As mentioned above, the 5703 levers allow you to route the derailluer cables under the bar tape, whereas with the 6604 the cables stick out the inside of the lever. Many people prefer the look of the under-the-tape routing, other people don't care.
The 6604 will allow you to downshift 3 gears on the back with one sweep of the lever, the 5703 will only allow 2 gears at a time.
The 5703 levers are designed with the new brake cable pull ratio, so work best with Shimano 5700, 6700, or 7900 brakes (although they do work OK with other brakes, there is a noticeable difference). The 6604 levers use a normal pull ratio and so work fine with almost any road brakes.
If I was choosing myself, I think I'd go for 6604 (Ultegra SL), but I can see advantages of each, and I'm not surprised they are a similar cost.