What are the pros and cons of using a provision for preferred shares with equity warrants in a VC term sheet?

Jeff, I'm not entirely sure of what you're asking...but will take a shot.
I'm guessing you're saying you've got a terms sheet with a VC for a series of preferred stock (with details surrounding that) and a kicker for either a direct common warrant grant, or a grant based on some trigger, again for common.

The first issue to address is "what is it worth". A preferred round is hard enough; yes, you have the pre and post money, but those are relatively meaningless on an exit. Look at your potential exits and figure out what the preferences take, and what is left over.

Pros include: A grant of common warrants is less dilutive than a similar grant of common shares, so on a share by share basis it might be a fair exchange for you. Additionally, if they exercise the warrants you get more cash, and that is good.

Cons include: If it is on this terms sheet, it will likely be on the next one. Do you want it there?
What value are you giving up? It is likely more, but more difficult to figure out.
Are the warrants structure so that you need to expense them if they are exercised?