ikea one is good. It's a bit bigger than most I have seen so easier for them to sit on and not miss. I have always used potty and training seat. If you are taking them to the toilet then training seat is better as you don't then have to clean the potty out but potty enables them to take themselves when climbing up on the toilet is a bit awkward when they are little. Also potty can be kept in the room you are in while they need to get there very very quickly. HTH
dd is small and i found a small one in morrisons for £1, it is small enough to carry out and about in pushchair. i had one downstairs and 1 upstairs as when she wants to go she wants to go dd has just moved onto a training seat after 2 months but is too small to get on and off herself even with a step so potty essential.
we have had all different potties! the best thing is to go by the size of your dc's bum! tiny wee bots need tiny pottys ect. we have the fisherprice one for dd4( shes just at the looking stage, not using yet), but we had a boots cheepy for ds4 , whos bigger bumed, and had various different for the rest of them.... some kids dont like them and do better with a seat and step .boots do a fab one with handles for the LO to hold on too