Another endorsement for the Larsen 2/70.
I recommend the open coil, as opposed to the closed coil version.
I have had friends say the closed coil cracked on them in the winter when they hit something with it.
I've had my open coil for about 5 years or so. Like it a lot.
The 'biggies' with mag-mounts are the strength of the magnet and the amount of surface area that magnet has with the metal it's attached to. That will determine the size of the antenna it will support and the amount of coupling to "ground". I haven't seen a mag-mount yet that was 'too big', but that depends on the use/abuse you put it through.
I think the mag-mount I have is a Larsen, but I just don't remember for sure. It does what it's supposed to do, and I'm happy with it. It is definitely capable of holding a typical 2 meter antenna just fine (a crowbar would be very handy to have when removing it).
(I can't say a mag-mount is my preferred method of mounting an antenna, but I still use them.)
I'm currently using an older antenna I've had for a while on that mag-mount. It's not a Larsen, it's a Comet CA2X4SR. But it does double duty for use in the commercial bands along with 2 meters and 70 Cm. It's certainly comparable to the Larsens though (got one of those too). Biggest 'complaint' is that it's 'stiff', doesn't bend much. So you have to take care where you mount it. Best in the world? Probably not, but it suits my needs just fine. It won't be going away any time soon.