UTG+1 is SmartFAB, a regular who I have at 9/8/5.
CO is 11.5/4/2.
I've got my usual tight image.

What's the best play here and why?

PokerStars No-Limit Hold'em, $1.00 BB (9 handed) Poker-Stars Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

MP2 ($22.60)
MP3 ($102.65)
CO ($97.50)
Button ($127.60)
SB ($109.10)
Hero ($178.05)
UTG ($102.80)
UTG+1 ($106.60)
MP1 ($101.50)

Preflop: Hero is BB with 6, 6.
1 fold, UTG+1 raises to $4, 3 folds, CO calls $4, 2 folds, Hero calls $3.

Flop: ($12.50) 6, 3, 4 (3 players)
Hero checks, UTG+1 bets $8, CO raises to $16, Hero ??