Sunnysider, interestingly I've done IM Wales (3x) and Cotswold 226, and my times are substantially different. Quicker at 226 than Tenby. However, 226, although flat(ter), isn't necessarily easier.
Swim is lake based, and easier to sight, but strangley, my times at Tenby are quicker, so there might be some tide/current assistance that I didn't fully appreciate at the time
Bike is flatter at 226, but this means there is no real recovery period, you have to pedal the full 180km, with little deviation. At Tenby, there are energy sapping climbs, but the pay back is some quick and longish descents
Run, Cotswold is flat, which makes it slightly easier on the legs, but Tenby is ideal for pacing and using a run:walk strategy. Walk the uphills,and run the downhills
I think Costwold is generally more 'friendly', in that it's not a scary sea swim, hilly bike and run, and cut-offs aren't strictly enforced. Still a 3.8km swim, 180km bike and 42.2km run though, and hard work.
As I've said before, the hardest IM is the one you are currently doing, or training for. The easiest, are the one's you've finished.
It is interesting. Once you're confident you can do the swim, unless you intend to be very competitive, you are better focusing your time on the bike, where you can get a significant improvement, but bike fitness will also enhance the run.
I think Gordo said that once you're under the hour mark for the swim, you need to swim train about 1,000,000m for a minute reduction.