I have the Doran system and am waiting for my next tirechange to install the sensors inside the wheels. Two things to keep in mind with the extrernal valve cap sensors on the cheaper units. They defeat the valve core. If they aren't screwed on tight and leak tested every time you add air, you could be creating your own slow leak. Also I've talked to a 1800 rider that had a flat because his valve stem broke. I've never heard of this happening to anyone I know, but there it was.
The Doran includes two 45 degree angle valve stems made out of metal so they don't flex. The sensor screwes on the inside of the tire to the valve stem. This dosen't defeat the valve core's function.
Now you can install the Doran system with the sensors on the outside of the tires if you want, and take the above described risks, so they have the only system I know of that can work either way.
If you do a search on TPMS here you will find other links on this subject.
Purchased the TPMS from Gene's Gallery. Just wanted a simple unit that gave me an "easy to view" air pressure reading each and every time I go for a ride. They have worked well, but I have only 1200 kms of use to date, so time will tell. They screw to the valve stem and lock in place with a reverse threaded nut.
Since this is my first such unit I cannot recommend this type over any other internal or external mount TPMS. The price was reasonable and gives me the info and peace of mind that I need.