Surbiton would be my preference between Surbiton and Thames Ditton but a) I'm biased and b) I'm not your husband ;)
These are the reasons:
More useful shops and facilities in Surbiton - if he doesn't have a car, then surbiton provides easier access to supermarkets (large waitrose), choice of dry cleaners, pubs, restaurants, stationers, banks, closer to sorting office, etc. This mightn't matter at all to him, but I find it makes life so much easier.
Much better and faster commute, notwithstanding the packed trains from Surbiton. If you time it right you can sometimes get a seat, but yes generally, the fast non-stopping service does get full, uncomfortably so at times. Nevertheless, I'd opt for speed rather than space and a longer commute for a number of reasons which mightn't be relevant to your DH (e.g., getting home in time for childcare).
Moire choice of housing tenure, lots of 1 bed flats in surbo and surrounds, particularly near the station. TD does have flats, but much of the housing stock is (big, expensive) houses and there's a much more limited rental market there.
Also, depending on where you live, Surbiton will potentially be easier to get to if you ever decide to visit at the weekend. It's on the main lines out to Hampshire etc. You can get to the IOW and new forest easier too, both of which are good if you're into cycling.
Re making offers on rental property....always worth a try but don't count on getting a reduction. Sorry to sound Eyeore-ish, but the reality is that there is limited housing stock and high demand.
Finally, you mentioned cycling. I don't know if you meant as an add-on to the commute, but if so.... be aware that you can't take bikes on peak train services into london unless they are folded. For that reason lots of people have Brompton-type bikes. On the plus side, if your DP can get a folding bike the options for commuting are much better. If he goes from e.g., surbiton into waterloo or one of the other stations into vauxhall/ waterloo, he can get to Victoria in about 15 minutes from waterloo and 20 minutes from vauxhall respectively. Season tickets to vauxhall are cheaper than into W'loo incidentally.