You've got two main options really in that sense, a red-dot with a magnifier or a magnified optic with a supplementary docter sight.
Using the 2 seperate items maximises the utility of the red-dot when not magnified, but you've got the weight of two bits of kit and it's not ideal when magnified. An ACOG/Spectre with a docter should weigh less (well, maybe not the Elcan) and the primary magnified optic is better, but the backup sight is pretty small and not quite as easy to use. Neither setup is cheap.
Just depends what you want really. I'd go with an ACOG w/ Docter since that'll be the lightest and you don't want to make an already heavy gun even worse, but that's because I've tried out all the options already and I know what I like. Most airsofters who don't use iron sights just use Aimpoints or EoTechs though.