25NL - Do I value bet here?

[phydaux]

One of the areas I've identified as a leak is that I'm not value betting enough on the river.

Recently I played this hand. I'd like some comments.

PokerStars No-Limit Hold'em, $0.25 BB (8 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

saw flop|<font color="#C00000">saw showdown</font>

<font color="#C00000">Hero ($31.35)</font>
CO ($14.90)
Button ($25)
SB ($36)
BB ($20.85)
UTG ($24.25)
UTG+1 ($22.40)
<font color="#C00000">MP1 ($20.65)</font>

Preflop: Hero is MP2 with 5[img]/images/graemlins/spade.gif[/img], 5[img]/images/graemlins/diamond.gif[/img]. MP1 posts a blind of $0.25.
<font color="#666666">1 fold</font>, UTG+1 calls $0.25, <font color="#CC3333">MP1 (poster) raises to $0.75</font>, Hero calls $0.75, <font color="#666666">4 folds</font>, UTG+1 calls $0.50.

Flop: ($2.60) 2[img]/images/graemlins/diamond.gif[/img], 3[img]/images/graemlins/heart.gif[/img], 5[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
UTG+1 checks, MP1 checks, <font color="#CC3333">Hero bets $1.5</font>, UTG+1 folds, MP1 calls $1.50.

Turn: ($5.60) 6[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
MP1 checks, <font color="#CC3333">Hero bets $3</font>, MP1 calls $3.

River: ($11.60) 7[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
MP1 checks, Hero....?

MP1's play just screams "Slowplaying a made hand." Was he slowplaying two pair, a flopped straight or a draw that came through on the turn or river?

Do you bet here, or check behind?