The S625X normally comes with the foot pod sensor. I'm pretty happy with the foot pod sensor. I have been running on treadmills at mixed speeds and paces and the foot pod has been consistently well within 5%. I recently bought two bike speed sensors and have verified that they work with the watch, but haven't put them on bikes yet. I may buy a cadence sensor or two. I don't have much faith in the accuracy of their power sensor on roads that aren't smooth, and it's plenty spendy too. Reviews on MTBR.com (third link below) have some very unhappy reviews on the speed sensors, so be aware of that, but I have a friend who has not had issues with his S725X picking up his bike speed.
As you can tell from the second URL below, the choice between the t6 and S625X (S725X is identical, but bike oriented due to the inclusion of a speed and cadence sensor instead of the foot pod). If you're mainly targetting biking, you may want to check out the Suunto t3 and t4, which appear to have many of the capabilities at a lower price than the t6. I got to demo a t6 this weekend. It is a thinner, better looking watch made with more durable materials, but I didn't like the feel on my wrist. While the S625X is bigger, it feels comfortable on my wrist, whereas the t6 felt like a big, flat metal plate on my wrist, which I found to be uncomfortable.
Cost is what drove me away from the t6. I got my S625X for less than half the price of what I was able to find the t6 for. I haven't looked much at the t3/4, and don't plan to now that I already have my S625X, which is working very well for me. I use mine with PC Coach triathlon software that I am using to prepare for a triathlon. If you want my review of that software, ask me in a personal message or on the triathlon team forum.
Sources of info on this super-watch realm:
PC Coach review of uber-sensor watches
Debate between S625X and t6
Bike review site feedback on Polar S710 (very similar)
'What better place than here? What better time than now?' -RATM
SW:185 GW:~165 - succeeded