I prefer whips, but here is my input from the top of my head.

Both keep your lungs healthy, help conserve herb, and are very efficient. Either type is definitely worth it.
"Browns" or the vaped weed leftovers are also great for cooking.

Whips are better for small/personal sessions (small & quiet). Power cord limits reach.
Bags are very convenient for group sessions. The power cord won't tie you down.

Whips take less than 2 min to go from bud>grinder>whip>vapor>lungs.
Bags take a little longer, but mainly because of the warm up/set up time and bag filling.

Whips, you can set it aside and resume your session whenever you want.
Bags, (like others have said) are somewhat limited with time, as I've seen bags have almost 25% of vapor condense within the first hour.

Whips you can precisely control how much vape you get from your herb. You can easily tell how much there is left to potentially vape.
Bags can be filled more than once from the same bowl, but it's a little harder to tell how much is left to vape.

Whips can have resin scraped from the glass. All parts are easy to clean (hot water & soap) and only need changing/cleaning of screens (which can also be reused). A $2 pack of screens lasts for a very long time.
Bags last for a fairly long time and parts are easily cleaned with alcohol. However, replacement bags and parts are a bit costly.