I have and use both. The Quarq works fine for a lot less money.

I did have to send it back to Spearfish when it was new however as it was not working properly. It has been reasonably good for the past two years though.

The SRM is slightly better in terms of spikes and spurious readings, but the difference isn't much.

I also have old SRMs. One of them was even old enough that it needed to be replaced due to microfractures. They gave me a good deal on a new wireless spider.

I also have old and new powertaps.

Everything eventually wears out. The Law of Impermanence. I'd go with whatever your gut tells you and don't expect it to last forever.