6mth AST + SPT vs 12mth AST

Serving documentation apart, from the landlord perspective what is the practical difference between a 6mth AST + ongoing periodic rollover and a 12mth AST with a break clause activating at 6 months ?

Break clauses are notoriously difficult to get right. Even those drafted by supposed experts often have adverse effects and claims from landlords based on them are frequently thrown out by courts. I would always recommend a 6 month fixed term followed by a periodic tenancy, preferably a Contractual Periodic Tenancy.