Hi,

We're just in the process of TT DS2. With DS1 we used a seat with a step incorporated into it and he learnt to sit down first (after mastering the potty for a few weeks beforehand). I remember another mum making a passing comment at a party once, that she was surprised he wasn't doing stand up wees? pfff. He eventually graduated to stand up wees, standing on a single step and I think we had less mess this way, as he'd already got the whole toilet thing and was taller by this stage. I found if you go out and there's a public toilet (disabled toilets seem to be really high) that stand up wees without a little step are impossible and it was much easier getting them to sit on the seat and aim downwards. There weren't any issues getting him to swap from sitting to standing - he just showed interest one day and we gave it a go with lots of high fives afterwards.
Each kid will be different. Our DS2 is currently refusing poos on the toilet and wants his nappy back on. We didn't experience this with DS1, but they're TT at different ages.
Good luck with it all