Just adding my own perspective to this. While the prize amount for a year long draw may be larger, I wouldn't be any more likely to donate (and maybe even less likely, or likely to donate a lesser amount or less times).Starting date Jan 1 with drawing on Dec 1
In a year long raffle, the raffle wouldn't factor into any donation decisions I made for the first 3/4 of the year, however closer to the draw I would likely consider the raffle and thus decide to make another donation to be a part of it all. I'd fear a year long raffle may lose focus, and have most of the donations coming in at the end. Multiple raffles I'd probably be more inclined to donate multiple times, again just to feel part of it all.
Alternatively you could maybe run 2 raffles over the year (at the lower prize value) and then an end of year raffle for a bigger prize but the tickets for it include all the donations over the year (or all the donations over a certain amount or something). I'm not sure how that would actually work, but it might incorporate the different motivations people have for donating/taking part in the raffle.