When I entered the profession in 2002, with a relevant degree, I entered as MTO1*** on the old Whitley scales, moving to MTO2 after a year to prove that I wasn't completely useless. The MTO2 post was matched to AfC Band 5.
Generally, no-one up here is considered for MTO3/Band 6 until they've been in the job for at least three years.
So to answer the question, one would take the Band 5 job to get experience to go for a Band 6 job.
I'd be interested to know if the situation is significantly different elsewhere.