The two biggest battery drains will be the screen (especially when on 50%+) and mobile data 3G/4G. Consequently the more items you have that need to sync, (widgets, email, calendars, social networking accounts etc etc) the more data you use, and the more frequent the pings to towers. These take effort, in short battery.
As far as notifications go, it depends on the setup. If your set to have email check every 15 mins, with a sound/vibrate/led/screen on alert, then of course that will deplete battery faster than just one form of notification. It's all variable and dependant on settings.
Generally though, the biggest burner will be your screen. Notifications in comparison will be a drop in the ocean, maybe a rather large drop, but a drop none the less.