If you really want to go the undersaddle route I'd recommend a Headway Snake or FEQ, both of which can be had in twin element versions - have a look at their website, then give them a ring - I've always found them to be really helpful. The sound from these pickups is excellent when well installed and settled in. I would recommend professional installation unless your are really sure you can cope yourself.
There is an endpin preamp on them which gives a really strong signal and a great sound often without the need for outboard eq. I usually use a bit more mid-cut either with an eq pedal, or at the pa mixer or combo amp. The effect on acoustic sound is a possible question mark - the elements are very thick, and have a softish coating. When I had one in my Lowden O25 I didn't detect any loss of acoustic tone when it was put in, but on removing it some months later after becoming suspicious of maybe a slightly weakened treble response from E and B strings I was rewarded with just a hint of extra sparkle and zing. Still excellent pickups though, and if mainly plugged in is your thing then they are a good fuss free way to go - I have an FEQ in my Fylde Oberon which gets weekly use plugged in for worship band and ceilidh band situations.
You could try soundboard transducers -
Baggs I-beam - fits to bridge plate with adhesive pads - easy to fit. Can be had as passive version which absolutely must have an external preamp. Active version is much better and easier to use - you don't need extra holes in your guitar as the preamp is (internally) part of the output jack. No controls obviously (though you can get a small volume control thumbwheel to stick just iside the soundhole), but an external preamp in addition works wonders. I've used I-beams active and passive in Lowden guitars - didn't like the passive version. The active version was reasonably OK for most styles and situations. Certainly a nicer sound than F*shm*n USTs. More feedback prone though.
Best things by far that I've tried are K&K minis. Really simple, 3 passive transducers Superglued to bridgeplate and wired straight to output jack. Easy to fit, very little weight added to guitar (no battery) and a superb sound. Although passive, the output is plenty big enough to go straight to amp or mixer. I usually use a K&K Pure XLR preamp though - it transforms the sound from very good to excellent. A truly "acoustic" sound representative of the unplugged guitar can easily be had. I have these pickups in my Lowden O25 and S10. The different sounds and characters of these two guitars are well represented when plugged in. Again, check out the K&K website - their stuff is remarkably cheap for the sound quality, and can easily be ordered from UK via the website - usually arrives within a week.
Hope this is helpful.