@ Unregistered - If you are mostly sailing in >10 knots, I would go with the R13 S--. In flex-stiffness, the R13 will be considerably softer than the R19 because the foil is thinner and has less area. The R13 is a very versatile fin and works in nearly all conditions and requires minimal tuning from the rider to get up to speed.
Remi is right, for lightwinds the R19 would be better, but if you were only going to buy ONE fin, I would think you'd get more out of the R13 across the different conditions. Make sure you go for an S-- as in the 160 it really appreciates a soft fin for power. This will be suitable even in strong winds and you still have your R13M if you prefer that as the wind gets up.
@ Ken - put your R13M in your board and put a straight ruler at the front of your fin from the bottom of the board perpindicular to the board. With another ruler, measure the distance from vertical to your fin tip. Like this photo: http://www.carbonsugar.com/wp-conten...08/04/rake.jpg
- although, measure the TIP not 15cm down from the tip like in this photo.
A Deboichet fin at +11cm is raked at vertical. So a fin at +8cm would be 30mm from vertical. +6cm = 50mm from vertical etc etc. That way you can find what rake your fin is regardless of its vintage.
You can change the rake a little by sanding the front/back of the fin head and kicking it forward in your finbox but changing the rake anymore than 1.5cm requires you to drill out your fin bolt holes in the fin head as they will no longer line up with your board holes.
There's some more info about what rake and stiffness does to fins and to your boards in an article I wrote here if this helps: