Through level 12 I can firmly say that a 6/6 monk-ranger split plays exactly like a 12 ranger when in primary Horizon Walker. It also seems that evasion works now even if you are uncentered. I was uncentered until L10 and evasion saves were triggering the whole time. Not sure if this has been changed and you can now evade while uncentered or if this is a bug. Updated feat choices were:
9 - IC Ranged
12 - Shot on the Run
12 - (monk) Toughness. There are three options here for your martial arts feat. Discipline to maximize your threat reduction from Shadowdancer, Deflect Arrows (although we get this in Vistani), and Toughness. It is likely that most will want to get the haste boost from Vistani and Deflect arrows is on the way to that, so toughness or discipline? Going toughness (and maybe epic toughness) this life and will try out discipline next time.
Now let's look at stances. Fire modifies STR so isn't much use to us. Earth gives us the all-important HPs but at - 2 cost to DEX our primary stat and slows us down by 10%. That's a high price to pay. Wind gives us the DEX boost but a -2 CON. Water is probably the most useful overall, we lose 2 STR that we don't need and gain saving throws, dodge, and +2 WIS. I don't have a lot of reaper points on this character so the saves bonus is useful. Ultimately, we are talking about fractions on the margins for all this stuff and maybe the 10% HP boost is the most noticeable effect you will get especially since you're likely already pushing dodge max from boosts in reaper and enhancement trees. As you can see from this stream of consciousness debate I'm having with myself there is no must-have stance so quality game design has left us with options. Another thought on speed - ideally you want to hang back and let other characters draw the agro. There are several reasons for this; you don't have much AOE kill capabilities and are ill-suited to be the lead zerger. Slowing down and scout snipering the most dangerous targets is what makes you such a bonus to a party and you'll still be able collect plenty of kills. I might be talking myself into Earth but then I remember it's a net 4 loss of DEX which equates to a -3 loss in damage from ability and DEX stance.