when should I separate slow-growing sweet potato slips?

I'm trying to start sweet potato slips for the first time--hoping to grow some roots for me to eat and lots of vine for my rabbits to eat (I'm in zone 5, upstate NY).

I have one sweet potato half in water and another in soil which have sprouted. The water-sprouted potato has had sprouts for 3 weeks or so; they leaves are a nice lush green but they're very slow growing--the longest sprout is about 4 in.long with 5 leaves, most are shorter. (The air temp is around 60, which I know is cooler than optimal; started these in a warm place over the fridge but moved them for more light when the leaves came out.) White roots are growing, not out of the sprouts, but out of the lower end of the potato which is in the water. I've seen articles saying to twist shoots off at 5-6" long at which point some of them will have roots...I do see little raised nodes on the underside of some stems and I don't know if those would root if put in soil or water.

Should I wait and see if the sprouts grow longer? Should I plant the whole sprouted half in a chunk so as not to lose the roots? Twist the shoots off anyway and maybe plant the rooted spud end separately? Any advice would be greatly appreciated.

I wonder if they are short because they are potato halves?  I use whole sweet potatoes and immerse about half of the potato in water, that lower part will send out a lot of roots and the top half send up sprouts.  I also start them where it's a little warmer and then move to a sunnier window when they start leafing out and that is always a cooler spot, like you mentioned.

I usually get quite a few long sprouts and if I start too early some are two feet long and can be clipped into several pieces to root and then plant.

Maybe they just need more growing time.   I started mine in mid February to plant late April here...so they took over two months to sprout and root.  A few of the later ones we just planted a couple weeks ago in early May.  They won't do much in the ground if it's still cool weather and I think it sets them back a bit.  

Maybe just experiment and pop off a few of the longest ones, root them (the whole length of the stem, up to the top few leaves) and plant them in the ground with just the top few leaves showing.  Until this year I always rooted in water and then planted.  This year I had some one quart planting bags I was given so I tried rooting them in soil in the bag in the hoop house where it was extra warm and I think it worked well....easy to plant and no planting shock  from bare roots.  Our problem this year is its been cool and rainy much of the spring and they really need some hot dry weather.

Hope there's something here that's helpful...kind of wandering.  I love sweet potatoes and they are probably our most reliable crop.  I've saved and sprouted from the same line of sweet potato for more than ten years and they are still producing as well as ever.