AST rent frequeancy - best monthly or weekly?

Other

When drawing up an AST is it best to set the level of rent in the agreement on a per month, per week or fortnightly basis?
Some tenants who have a per mth agreement often pay each week or 2 weekly
Depending on how it's set, does this then affect S.21 or S.8 notices for example? Also how does each rent frequency affect things when then moving to a stat periodic tenancy?