Thinking I need an ar pistol. Trying to decide between 7.62x39 and 6.8. What's the best barrel length?

Best for what purpose? Details please.

I wanted a 7.62x39 pistol, but I didn't really need one.... otherwise it would be finished now.

Range toy and maybe 100 yard or less deer hunting. We have a special pistol season that I could use it in. Trying to justify if myself. Kinda want but 556 steel ammo isn't much more than the x39. And I already have mags. Would sbr but don't want to mess with walking papers if I go out of state.

so I built my 7.62x39 ar pistol for multiple purposes.
1) went with a 10.5 inch barrel due to 150-200 fps lost in 6 inches less barrel vs 7.5 inch loses almost 300-350 and doesn't get a full spin in a 1-10 barrel
2) with an extended kak sig brace buffer setup you will be 27.5 inches from muzzle threads to buffer tube end which will be legal and over the 26 inch rule for legally adding a vertical foregrip.
3) I just submitted my paperwork for a form 1 suppressor and an 8 inch suppressor will queit it down a lot and keep it a reasonable length when moving with it being that it will be 18.5 inches or so but my friend has a 7.5 in barrel and is running a 10 inch suppressor and will be a inch short and quieter maybe.
4) I have shot my 10.5 upper with a micro 2 moa primary arms red dot at 125 yrds standing and can get a good group around 2 moa
I love the 7.62x39 round in an ar there is a lot of info on other posts just search it

6.8 has a following also and I know there are great reports out there
just made mine to run cheap ammo suppressed and legal to hunt with

If its primarily a range toy, I'd stick with the X39 and under 100yds the barrel length won't matter. Just add a linear brake or similar to send the blast down range.  
I went with a short barrel 7.5" because of the fun factor.

6.8 will cost more to shoot. If you were primarily hunting I'd go 6.8 with a longer barrel. I went 14.5" light weight 6.8 barrel with pinned comp so a stock could be used while hunting.

^ that would be a rifle not a pistol. He wouldn't be able to "pistol" hunt.

X39 in a ar is really fun! Cheap to shoot.  It may take some work to get it 100% reliable though.
Do you reload? Have you thought about 300 blackout?

11 inch!

Just providing the reason behind the barrel length on my 6.8... agreed it wouldn't fit the OP's purpose.

Yes I reload. The blackout is kinda interesting because I can use my mags and the extra parts I have lying around. Is it effective at <100 yards?

Technically a pistol can have any length barrel. Depending on the state and hunting regulations that may be different.  For example in MN a pistol is designed to be fired by one hand (i.e. no VFG or stock) and either be less than 26" OAL or have a barrel less than 16". Under federal law your pistol could have a 100" barrel as long as doesn't have a stock.

OP I recently faced a similar dilemma and went 300blk. I already have a 5.56mm pistol. My argument against 6.8spc is to get a reliable mag it seems you have to spend $35 a mag. I thought about 7.62x39 but again non standard mags and non standard bolt. 300blk keeps everything the same is very effective out of a short barrel.  The downside is ammo cost. The cheapest I have found for range ammo is $.50 a round you can reload it for much cheaper as the parts are standard. 223 brass and 308 bullets. 6.8spc also has a non standard bolt same as the 7.62x39 bolt I believe.

Rogue how are you liking your pistol?

8.5 inch... Green mountain is making a run of them.

Well I went off topic and have a 277 wlv barrel on the way. 10.5" just like using the mags I already have.

Oh I think you will have some fun with that, especially if you reload.  Been looking at the Wolverine for some time and it has some very good points to it.