Looking to upgrade wort chiller

Twenty months ago when I caught the homebrewing bug I started doing simple 5 gallon extract recipes using a 25 ft copper wort chiller. Last spring I moved into 5 gallon all grain brewing and this year I want to get into larger size recipes now that I have my keezer project working.

Thus, what size of wort chiller would be ideal for doing 10 gallon size brews with a keggle? Anyone have opinions on coil diameter and whether copper or stainless steel is better?

Thanks...Mick

50' 1/2" copper works really well in a 10 gallon boil.