Male and female rabbits each have advantages and disadvantages, although much depends upon the temperament of the individual rabbit and early 'bunnyhood' experiences.
Males tend to make much better pets, bond with their owner and can be much more affectionate. However, they can also spray urine and are more likely to become amorous with brushes, fluffy slippers and soft toys! Two adult males housed together are likely to fight,
Pubescent or un-mated females can become irritable (bunny-'PMT'?!), have false pregnancies (whereby they build a nest and pluck out their fur) and may dig a burrow in your garden.
However, these are only generalisations and their background, breeding and personality characteristics are much more important factors when choosing your new pet(s). Puberty can be as early as 3-4 months in a male - so please keep a close eye on his nether regions!!
Adopting a pair has definite advantages. Rabbits are social animals and enjoy rabbit company. It's wonderful to watch them interact together, they have a constant companion and young rabbits settle in very easily after adoption. You do, however, need to choose a pair or a partner with care. Two males together can fight while a male and female together will have unplanned litters (potentially inbreeding) and 2 females can have more false pregnancies than solitary females.
The second pairing choice would be 2 sisters but, if you are choosing just one bunny for now with the intention of adopting another later, then we'd recommend buying a boy first. He could then settle in and be castrated at 3-4 months and then meet his girlfriend shortly afterwards. If a buck is castrated as a mature adult aged +5-6 months then he will may still be fertile for several weeks following his ‘snip’ so must not come into contact with any females for 4 weeks.