They are actually using two different chipsets. The m1200 uses the BT7-XXX series chipset while the 240 uses the SiRF III. Basically the 240 is smaller and will give you fast tracking and multiple updates per second. The m1200 is bigger and has better battery life and can track more satellites simultaneously, but the updates are only 1 per second (side effect of power saving).

They are both good choices, but I prefer the 240. The major advantage of the 1200 is that is works will indoors and in tricky areas (eg manhattan). I modified my 240 with an external antenna, so I get pretty great signal while driving. It is a tricky procedure, unless you are pretty good with electronic circuits.

All in all, I think you cant go wrong. They are both great products.

Cheers,
James