The Twin Cam Ford-based engine is far superior. When properly tuned the TC is reliable, tractable, gets excellent mileage, & has 126 HP. The only downside to a TC is that a water pump change mandates an engine removal. I have performed this several times by myself with a lift, but it's easier w/ another pair of hands.
The TC engined cars (1972 onwards) are also better built, slightly more comfortable, etc.
I do not consider this to be a close call at all. Hope this helps!
I've owned two of each (my last TC attached), and largely agree with your assessment.
BUT, the TC is a more complex motor, and with complexity comes, well complexity. In my experience they needed more attention. Putting a Pertronix on my last twin cam Europa was transformative.
I believe Bean made a 'cassette' water pump which, once you got it in, made future repairs much, much simpler.
The five speed...well it's always nice to have 5 speeds. But climb into one and start driving, and you'll do two things: wonder if you can even drive a manual transmission and wonder if there is something wrong with it. Read reviews of when the 5 speeds came out. In brand new cars the reviewers were decrying the shifter. To call it imprecise would be an understatement. You literally have to learn the car. In both of my TC's, once I got used to them, I was able to be quite precise and quick in my shifting, but it took time. You don't guide it into gear, you have to deliberately place the shifter exactly where it needs to go.
And it's made worse if you drive one in which the several bushings in the cumbersome shifter links and rods have been allowed to wear. Even when the entire assembly is in good fettle with all new bits, and adjusted as best as can be, they are a bear to get used to.
One other thing, on the five speeds, the plastic gear in the transmission that drives the speedometer cable are prone to self destructing, and they are made of unobtainium. If you find someone with one to sell, he will likely know how rare it is and will ask many $$$ for it. When mine went, I had a GPS Speedo built for a couple hundred bucks that looked almost identical to the Smiths original. Since then, I've heard of a GPS unit that drives the actual speedo cable, meaning if your OEM speedo is working, you can still use it. I'd go that route now.
The TC's are obviously quicker, and do have more room if you're on the tall side.
If you're just bombing down some twistie backroads, a well maintained and sorted Renault powered S2 will give you a lot of grins. The S1's main drawback is that the body is literally fiberglassed onto the frame, so if you ever need to do any body off work, you literally have to cut it off the frame.
As for the later cars being better built. Well. Yes. But we're talking vintage Lotus. It's like saying a low tar cigarette is healthy for you.