Karrueche Tran vs. Rihanna, sexiest Instagram PHOTOS of the week: Chris Brown and Drake 'battle of the girlfriends' continues [POLL]

Are fans ready for a Rihanna vs. Karrueche Tran Instagram photo showdown?

The girls are considered Hollywood "frenemies," although they rarely bump into each other very often. But what can you expect when both ladies dated the same man MULTIPLE times? We're talking about Chris Brown here...

Anyway, RiRi just celebrated her birthday on Thursday (Feb. 20), so in honor of the (now) 26-year-old songstress:

(Instagram)

Pretty hot! But now check out Tran, who's finally excited to see her main squeeze out of rehab:

(Instagram)

She's gorgeous, too!

Although Breezy's still going through some major legal battles of his own, he must be pretty proud of having dated two such stunning ladies!

So - which beauty had the better week on Instagram... RiRi or Tran?