which type of Deposit scheme

Tenancy Deposits

My rent now falls in to the deposit scheme . I deal with 4/5 european students on an joint AST with parents as guarantor and take 2 month deposit £4,290 to avoid non payment at end of year . I like to off set the last month rent if all is up to date . I've poured over the 3 choices, confused as which suits best - my feeling is the insurance . Any advice, advantages or disadvantages welcomed . my only proff body is as an accredited landlord and being a member with RLA . ths