I'm in the UK too, Trot, and I have direct experience of 3½

devices:
1) Power Commander III
Ran that on my 103" CUSE. Fine I suspect, IF you can find a tuner who can tune them properly. I had mine done at one of the Dynojet authorised tuners, and it was awful. Ran worse than when I started, and it wasn't cheap.
2) Thundermax with Auto Tune
Ran that on my 110" CUSE
2. An unmitigated disaster. Main problem is, assuming you can get a base map close enough, even though it may self adjust to seem OK locally, if you tour - and like me, I guess you go to Europe, where it can be very hot, and very high, you WILL have a problem. The TMAT does not use the ION sensor, and if very hot or high, your 110 will pink (what we in the UK call pinging, for American readers) like there's no tomorrow. Crossing the Sierra Nevada in Spain in 07 I thought there was an army of blacksmiths hitting anvils with big hammers inside my engine! Remember, that the TMAT cannot auto-tune the timing, or make timing adjustments when it starts pinking - it does not know that the engine IS pinking, as it does not use the ION sensor!
3) SERT with (here comes the ½ device!) the addition of a Head Quarters Pro Tuner. You still have a problem with the SERT (or in your case, it would be a SEST now) in that although the MoCo's base maps are close, it really needs a decent tuner with a Dyno to make it as good as it can be - see entry under PCIII, above - though if your configuration is close to that for the MoCo's base map, you could live without that - what I've done up till now. Now that at that point is just a different map, loaded into the existing ECU. What that can't do well is any great range of auto AFR adjustment, to suit differing conditions. BUT - at least it still uses the ION sensor, so pinking is minimised. That's where the Headquarters Pro Tuner comes in. It's additional to the SERT, still lets the standard ECU use all of the sensors, but auto tunes the AFR to an optimimum level. For a start that lets your motor run cooler. I only got the HQPT close to the end of last season, so I have only 500-600 miles with it installed, and can't fully comment on how good it is, but early signs are good - for a start, the 110" runs noticeably cooler than it did with just the SERT map loaded.
Finally, the HQPT is a failsafe device; if it stops working, your ECU reverts to normal operation with the loaded map. If the TMAT fails when you are away touring, the bike is going nowhere fast, and you can't just carry the MoCo ECU with you to swap, as you'd need the O
2 sensors changing too, which (at least in my case, a CUSE
2 with V&H Touring Dual headers) means removing the headers, and if one of the sensors is seized - as one of mine was - you are not in for a fun time!

Hope that's helpful!
Jim