I am going to pick up some HSM 30/30 to plink around with out of my Marlin 336. I cant decide between 150gr and 170gr. What should I go with and why?

For single thickness paper punching I recomend 150 grain.
For five gallon buckets and dirt berms I recomend 170 grain.

My Win. 1894 liked 170gr... I would use the new Hornady Leverloution (or however it is spelled) for hunting. I was very impressed with that round...

Rmpl

If you get in the killing mode with it deer in the neck or lungs/heart like 150 SP.Bear if you fancy sitting up a treestand over a pile of old groceries they like 170 is about required and nice shot placement.Bigger bears need bigger guns in my estimate.I got some 180 grain Imperial soft point bullets some years ago from Canada when they quit making ammo to send to the USA and load them in my cases if I draw a bear permit and decide .30-.30 is the gun of the day.Coyote like the 150 grains also as they always stop for it.I am not much of a paper puncher in the old time hunting rounds but just like to shoot paper enuf to make sure the gun is zeroed.If you do not load and near a Kmart some of the stores got red box Federal 150 gr. marked to $10.00 a box which is a good price these days

I always went with the 150s because of the little extra velocity.  For plinking I would just get the cheapest.  However,  I think the LeverEvolution is now the gold standard for hunting apps, it is only made in 160.  

Sometime back, I did a bunch of factory and handloaded accuracy testing with a particularly accurate 30-30.  The 170's (in several brands) were almost always better i.e. no 'slight' flyers.