I did Londolozi (which is next door to Singita) last August. Advantages of a winter safari are:
- dry season
- dry season means low grass and fewer leaves on tree so it is much easier to see animals, particular smaller ones like leopard stalk in the grass or lazying in the trees
- sun comes up later so a later morning safari start and more sleep! Winter safaris get going about 6:15 versus 5:30 or 5:45 in the summer months. Your afternoon safari also starts earlier to fit in you sundowner at the appropriate time. You get to go to bed earlier too although Singita has fairly rigid meal times that the safari trip work around. At Londolozi, they then to play it by ear, (i.e. if you see a pride of lions on the way back, the ranger stops the jeep even though chef will not be happy given guests from different jeeps are turning up at various different times).
- if you are concerned, fewer or no snakes - they are all hibernating
- much fewer insects and no mosquitos

Disadvantages of a winter safari
- we did brave the pool (unheated) if only for a grand total of 1 minute.
- you will need a fleece for the first hour of the morning and the last hour of the evening one (but be ready to strip for warmer weather after 8/9 am or so with some thinner clothing underneath)
- short time for an afternoon nap

I wish I was there again. Enjoy!