While the pay is not bad, the company is openly hostile to any leave requests and asks for a ludicrous 2 months notice even if for a solitary day or even a few hours. If you are primarily employing young people who are keen to get to know Japan give them just a fraction more leeway to enjoy the visit. The full-time contract is a better deal but you will work weekends and you will work nights and you will have HUGE gaps in your schedule with nothing to do and the expectation that you either don't leave the school or don't wander out for too long or too far, even if you have 5 hours to kill before your first class (and this does happen from time to time).
As to the part-time contract. Also pays better than much of the competition but with two big catches. The first being that the ¥3000 hourly rate (¥2500 p/h with ¥500 p/h completion bonus) hinges upon you finishing the 6 month contract and second catch being that if students cancel on you, you don't get paid for that hour. Meaning that you could regularly be travelling for longer than you are working, and can be stuck in the school from 5pm to 10pm and only teach one or two classes.
You will be at a different school, sometimes with wildly further travel times, most days. The logic behind this unknown, perhaps to keep teachers uncomfortable and unable to poach customers as private students?
The textbooks are idiot-proof and better than the previous editions although horribly overpriced for the students - who seldom stay longer than the initial sign-up period.
There is a largely morose feel to most of the schools and the district managers in all the districts I've encountered are overworked and miserable, and they don't hide it very well. This may have something to do with the outrageously outdated systems. Not a single school I went to had internet, and many don't even have a computer. All paper and fax, I'd check that payslip if I was you, especially if a part-timer.
Essentially, like most eikaiwa, you're an English speaker with some personality (and you really don't need that much) for the ESL industry conveyor belt. I will say for Shane that they are slightly more selective than the other big chains, but that's not really saying much.
I'd go with ECC, Berlitz or AEON if I was you, but I've stuck out Shane because I've been here long enough now to get a slightly better ride, but it took a long-time and is frankly not worth the wait.