If you want to use the same duvet in winter and summer you`re better off getting one that is two duvets that will fit together. Ay 9 tog and 4 1/2 tog. You can put them together for winter and detach for summer. I don`t think the brand makes much difference. I bought a 7.5 tog duvet recently as my local market. They`re all much of a muchness unless you want to finest goosedown.
Depends on where you live and whether you have central heating and how you use it.
We have duvets of 3 different Tog ratings and a sheet for the summer. 15C is comfortable with a tog of about 10 for 2 sharing a bed. 10C 12 or 14 tog it very much depends on the person.
the supermarkets all have good priced duvets. have recently binned all the duvets i've had for years and years and years, and treated us to new ones from asda. they're not actually for us, we prefer sheets and fleece blankets, but for the spare beds. personally, i wouldn't spend a lot of money on a duvet when you can buy perfectly good ones for very reasonable prices in supermarkets, markets, outlets, etc.
You can buy duvet's that are fastened together with press studs so that in winter it will be warm,and then in summer you can seperate them so that you will be cool, Most large stores sell these or you m,ay even find one on Ebay.
Hope this helps
I recently bought a feather and down one from Argos, it cost only about £25!.........it's lovely, think it's a 12.5 tog, I'll just add a warm throw in Winter if needed..........bargain!...........