A huge number of reason to choose EX over the HPX300.
1/2" chips is one. If you have the budget the PMW-350 has 2/3" chips.
Also the codec is more efficient (longer records for example).
Also if you have an fast laptop with an Express port you'll get very fast offloads.
The option to use affordable SDHC cards is nice too.
Stick a Nano on it and you're getting 4:2:2 out of the SDI port as well coming from those nice 1/2" chips.
I'd get a demo of both before making a final decision. They are IMHO evenly matched and a lot comes down to personal preferences. For example I happen to think the Panasonic ergonomics are better than the Sony. Also consider the workflow- P2 vs. SxS.
The HPX has 1/3" chips. This means that you have to use apertures wider than f5.6 to avoid diffraction effects from softening the picture. The EX with it's 1/2" chips is good down to F8. The EX can use a range of media, either SxS,Memory sticks and SD cards with official support for SD coming very soon. The HPX can only use P2. If your going to use the HPX at AVC-I 100 you will need 3 times as much media as you would with an EX. A 16Gb card in an EX lasts around 50 minutes, but only 16 minutes using AVC-I 100. Archiving and backing up 100Mb/s material also takes a lot longer. Of course you can use AVC-I 50, but IMHO that is not nearly as good as the EX's 35Mb/s.
I find that the HPX has a lot of fixed pattern noise which is clearly visible during pans or when objects pass through a shot. The EX isn't noise free but I prefer the almost filmic, moving noise of the EX over the fixed pattern noise of the HPX.
They are also two very different designs of camera. An EX would be great for shooting in confined spaces such as in a car, while the HPX may be easier to use on a tripod. Are you working with clients that expect you to turn up with a full size camera, or is a small format camera acceptable?