another vote for the SRM. There are some units out there that's probably 20+ years old and still functioning. A friend has a unit going back to the PC-IV days. When it comes to dependability and accuracy, there's no one better. I got soaked in a driving storm this past Monday, and the offset (think of what a scale reads when nothing is on it) reads 396. the next morning, i check the offset when the unit is totally dry and it reads 392; essentially a difference of 2W; the day before, it read 398; basically, very stable. OTOH, my quarq drifted the equivalence of 10 Watts within the span of 25 minutes, and it was raining the whole time (as opposed to going from wet to dry or vice versa). No idea how much it would have drifted if i went for 60 minutes. My unit was good enough in the sense that i had issues only in the wet, whereas others had worse issues (drift of 20-30 W) in dry conditions.

If there's one PM that you can be sure it'll be there whether or not you are still riding 10-15 years down the road, it's a SRM.