Hey! First; congratulations! I am sure you will have an amazing time :D
I LOVED my time at Tamadaira. Yes you do have to share bathrooms (there is like 20 private shower rooms in the hall though, so you never have to wait, and they get cleaned every day). Toilets are shared too, but that is only in your unit, meaning you are tops three people sharing that and a sink (and then you have a common space where you can store shoes and bigger things too.
You also share a kitchen in Tamadaira (also cleaned every day), but some units have a kettle and mine even a micro oven so you can heat that. Seiseki did have their own kitchen and laundry rooms, but you had a curfew and also video surveillance that was checked regularly. Personally I do not care for being watched THAT much on. The landlady checking in once in a while in Tamadaira was enough for me haha.
I do not know this for a fact, but due to the big amount of shared space, we in Tamadaira got very close (the ones that used the common space areas ofc), and we also had a lot of freedom to go out and get back late. We also could throw some parties without any restrictions really. Also more often than not, other exchange students came to our dorm to hang out, so we got to socialize a lot with a bunch of different people, both Japanese and not.
— on a side note: not every resident used the shared facilities or interacted with us, so it is also possible to be very private in Tamadaira too —
I think choosing between the two boils down to how much one can handle sharing. I personally am a super bad share’er, but some how I managed to get though it anyways. It was nice on off days, you could still run into friends and talk to somebody, as opposed to living in an apartment par example. You do have to share a lot, but it also forces you to talk and read Japanese in order to work the appliances or get help with it haha.
Hope this helped, and please write if you have any more questions!
(btw you do not have to buy toilet paper in Tamadaira, they refill it for you. I did not know this, so brought a lot lol)