I have used both Helix's and Herpstat's over the last 11 years and Herpstat hands down.
No competition.
Helix's technology was amazing, the best out there in 1999.
Problem is, they've done nothing to improve their product since.
EIDT: Sorry my mistake, they added a grounded extension cord a few years ago. Groundbreaking!!
I just got my first Herpstat Pro, and the Helix isn't even in the same league in my opinion. Little more expensive for the Pro version, but you get 4 thermostats for the price of three while only using one wall outlet.