IMO, I don't agree..

600 fill down is nice if the fact that it gives more loft for the weight than, say, 450 or 500 fill down, actually translates to *less weight*.

I would not choose a *heavier* bag because it was down, or a higher fill number.

(I might chose it for more loft, better fit, or real warmth)

the synth bag is interesting because it is so light. however I am slightly suspect of the rating - see if you can find someone
here with some experience in it.

My only real reccomendtion? buy the kelty and the marmot at REI, try them on, and return the one that doesn't fit as good. For me fit is real important - how it feels.

I'm at a loss to give you a better recommendation however because

1) I don't think I own a 45F rated bag.. my lightest sleeping bag is -3C rated. (so like, 25-28 F)
2) Most of my bags are twice or more your budget I spend as much as I can tolerated here, and I'll cheap out everywhere else.