You want a weighted base with integrated sticky pad. Sits on dashtop. I love mine. The 1520 is right there for your HERE Drive + HUD and your eyes never leave the windshield. Traveled extensively with this one and it never slips, never drops the phone. Bombproof.
Note: It's pricey because you have to buy the phone holder and the dash mount/arm separately. It's worth the $$. I've used the 1520 holder for over 2 years in my 2007 Toyota Prius and love it.
(EDIT: Sorry for resurrecting what may be an outdated thread - I hadn't paid attention to the date of the last post in it as I was just browsing the 1520 forum and saw this thread.)
I've never had lasting success with a suction cup attachment, especially with 1520. The phone is not only heavy, the size gives it a lot of momentum to compensate for as it has a long "lever" to counteract the suction.
I've had 2 different weighted base mounts w/ integrated sticky pad on the bottom and neither has ever moved on me.
I have the ProClip (aka Brodit in Europe).
Have had them for years and would always recommend them.
The only issue I've had is in my current car, where I opted for the A pillar mount, with the 1520 so big it only just fits in the gap.
I bought the Arkon SM517 back in December 2013 and it's been great for me. Only issue is that they don't provide enough attachable legs in case you lose one, but I just ordered some from the Arkon website for free.