As a CM I would say go with your gut instinct. I would arrange to see them both again, this time with hubby en tow. He might see things in a different light?
Re the flat vs house. It really depends how the space is utilised within the space.
Re the setting.
I have a two bed spacious flat, one bedroom is a dedicated playroom. The open plan diner/kitchen/lounge area is also a play area. My friend, who has a 3 bed house, doesn't utilise upstairs at all, and has made a space for a play area in the dining room. My mindees have loads more personal space in my home, even though it's a flat.
Re the garden.
I have a communal garden (am in SW London), which is huge with big trees... we are in it as much as poss - we take down balls and games - granted there is no sand pit/trampoline, but the playground and common make up for this I think. I have a park (with a toddler playground area) 2 mins away, local shops like PO, vege shop etc. 2 mins away, Wimbledon Common is across the road. My friend has a garden and she is the first to tell me they spend lots of time there but she doesn't get out much with the kids due to having her own garden.
We both do toddler groups, library visits etc.
Re school runs - I did one briefly and then stopped as it interfered with meal and sleep times too much for the younger kids. My mindees (4 of them - I have a variation in place), are under 4. I now do not do them.
So when visiting the settings again, see how space is utilised inside and out... and I do think it's a good idea that fresh eyes (hubby's!) see it then too, you might see different things this time too