I haven't handled a Montbell sleeping bag that was as well filled as a Western bag. Apples to apples, Westerns are more densely filled, ie no cold spots. Most people will find the Summerlite more accurately rated of the two. Although a cold sleeper, I've used the Summerlite (w/adequate pad insulation) below freezing happily.
If, by chance, you're a bigger guy and want more room in a bag, the MegaLite is the wider cut version of the Summerlite. Or you could get the expander panel, but probably just go w/wider bag.