DD ( 2 in March) in cot at home but on Mnetters advice as at grandparents for 2 months we ent cold turkey into single bed with a bed guard. 1st few nights I slept in room with her ( OTT panic) No problems at all . She still has her grobag for nights and after 1 month we have done naps in 'covers'. So far she has only got out of bed twice. On our return home we have brought forward order of new bedroom so she can have a single bed asap.
Single bed if you've got the space. We went for toddler exrendable IKEA bed as DS's room is very small and will have to fit DD in as well before too long, but given the space I'd have got a single or even a double (less likely to fall out, plus useful if you need to fit in extra houseguests occasionally).