For downhill / alpine trail riding you really need specialist cover IMHO.
I've used Dogtag a few times, and had cause to make a claim. Very good they were, loads of Insurance states Moutain Biking under sports they cover as standard, but the small print has loads of exclusions.
Used Snowcard last year and Dogtag this year. I've not had to make a claim against any policy.
IMO the important thing is too make sure you and your kit is covered for an airlift off the mountain. If you can't decide which level of cover you need then call the company and ask.