What it comes down to now is your finer preferences.
Jack Martin is £45 a week
more expensive than Rootes. Think of what you can buy (more specifically, how much alcohol you can buy
) with £45 in a week. This will generally be people's deciding factor in arguments like this, but we can look deeper if you so wish.
For that £45 extra per week, you do get an ensuite room. This, as you probably know, has it's pros, and very few cons outside of taking a chunk out of room size. Rootes has a basin in each room, so you'll need to share a shower/toilet with the other people in your hall.
The number of people you will be sharing a kitchen with varies between 12 and 19 in Rootes. If you get stuck in the 19 kitchen, it will be great fun no doubt, but then it's far more likely to be a mess. Jack Martin sticks to 12-13 per kitchen, so it's more consistent. However, this may mean a slightly smaller kitchen to accommodate (as you would expect).
Rootes has a more socially-accessible layout. You can access vast swathes of rooms once you're through the front door, so you will always have people there. The same is true of Jack Martin, but there's fewer people to each block.
The last thing to consider is let length. Jack Martin is only 30 weeks whereas Rootes is 39. This means you'll need to bugger off out of Jack Martin before 10am on the last Saturday of term, making the end-of-term party a little bit of a no-go if it's the only time your parents can pick you up/you can get a flight etc. There's very little storage space in JM for the holidays.
But you're right, both are well located and both have a good atmosphere, so there's not too much to distinguish them!