Limo Quote- which is better?

For a Hummer- $800 + 20% gratuity (=$960)for 3.5 hours (we'll probably need 4)

For a Hummer or an escalade (they have both available)- $950 for 4 hours- gratuity at our discretion


I can't figure out where I'm getting the better deal. . .would I still be expected to tip 20% at my discretion?