Mini-Britney vs. Britney -- Spears' Vegas Act Faces Little Competition

Britney Spears is already picking up some Vegas hangers-on … ‘cause Mini-Britney is launching her own rival act on the Strip -- and TMZ has learned it’s opening the same night as big Brit's show.

Sin City sources tell us ... Mini-Brit, aka Elena Gant, is steppin’ up large ... kicking off a new show at Beacher’s Madhouse in the MGM Grand Hotel on Dec. 27th… the same night Spears opens at Planet Hollywood.

And 12" isn't the only discrepancy between the ladies (5'4" to 4'4") -- Britney is set to rake in about $310,000 per show … while mini-BS will make a slightly shorter stack of $310 per night.

Still undecided about which show to check out? If you order an expensive bottle of booze at Mini-Britney's gig … a little person flies out of a stuffed elephant’s BUTT to deliver it to you.

Stay classy, Vegas.