I have a DC who attended one of these schools and I have friends with children at the others so will try to give some balanced thoughts based on my experiences and what I've heard.
Firstly, as you will know WCPS and the Rowans are pre-preps and only go up to 7. The 7+ process is a PITA imo, unless you have an unusually compliant child who is fairly academically gifted and likes sitting practice tests. The schools help prepare of course, but you as a parent will either end up doing a lot of the work yourself or you'll pay a tutor to do it if you're hoping that your child will get into any of the highly selective 7+ preps (KCJS, St Paul's etc.). The big advantage of Rokeby, therefore, is that you avoid this process and (depending on what senior school you end up targeting) your child could have a fair degree of continuity in their education until 11/13.
Both WCPS/Rowans are fairly demanding and fast-paced in terms of what the children cover, with a relatively heavy homework burden for children that age and you'll need to be fairly involved as a parent. I'm sure at Rokeby you'll be expected to be involved as well, but since they're not preparing for 7+ assessments, the load seems to be a bit lighter. It depends on your child's personality and ability how well they will cope with this, but there is little joy to be found in pushing a reluctant child who is out of their depth too hard.
If you're aiming for your DS to go somewhere like KCJS/St Paul's at 7+, WCPS/Rowans are probably better bets. That said, the 7+ assessment process for both of these schools is absolutely brutal for children of this age (Kings especially) and you may find as the time approaches that you rule them out as options because you're unwilling to put your child through this process. Again, this will depend on your child's abilities and personality.
WCPS and Rokeby are single-sex and that may or may not appeal to you as a parent. If your DS goes to WCPS and then gets into Kings, for example, he could go through his entire education without ever sharing a classroom with a girl. Different people will feel differently about this. Rowans has some girls but is majority boys because the 7+ options for girls in Wimbledon are quite limited.
Where do you live? Tbh the morning commute to school would factor quite heavily into my decision-making process were I in your position.