R621 has rail joiners fitted and the rails are relatively loose in the sleepers compared to R8090 which is stiffer (and more easily broken IME). Peco Streamline is a better buy - R621 is £4.99 for 970mm less whatever discount you can find, R8090 is £4.99 for 915mm, Peco SL100 is £3.90 for 914mm and is also available with concrete sleepers (SL-102 £3.85). The Peco track has no track pin holes; their track fixing pins (SL-14) are very fine (and easily bend if hammered into hard wood); alternatives are double sided tape or glue to hold it in place or alternatively drill small holes for Hornby like track pins.
If you bend flexi it tries, quite successfully, to straighten out. If you bend semi flexi it tries to straighten
a little bit but tends to hold the curve. Personally I don’t like it because I found the rails sometimes came adrift from the chairs.
My thought is if you have never tried flexible track have a go with the semi first then move onto ordinary flexi.
I remember when, as a lad and first using flexible track suddenly realising that I would have to trim the length of the rail on the inside of the curve!