They each have their advantages and disadvantages as far as forming ammo etc., but is their a clear winner in any category? How similar or dissimilar are they ballistically?

Good question. Hope this gets thoroughly wrung out as these are very close together.

Well, I'm in no way an expert on either, but the idea of using a 223 bolt and magazine seems like an advantage to me. I'd have to let the guys with first hand experience comment on performance though.

True on the bolt and magazine, but if you own a .338 Spectre, you've already got the magazines...or an excuse to buy a .338 Spectre.

I'm willing to bet there are more 223 rem AR15 mags flaoting around the country than just about anything else...at least according to that CNN report a couple weeks ago...

I sure as hell hope so ;)

BTW, I'm pretty sure this barrel is the first one chambered for the .30 HRT...

Quoted: They each have their advantages and disadvantages as far as forming ammo etc., but is their a clear winner in any category? How similar or dissimilar are they ballistically?

My main experience is with 300 whisper, but thats not too far from the x40.

Ballistic comparisons are a little iffy since there is limited data.  the HRT has more case capacity, and that should lead to more velocity-- all things being equal.  This would also mean you could use slightly slower powder.  The other variables will influence the numbers more, like a 16" HRT would probably very closely match a 20" 7.62x40.

Both should have a preference for shorter bullets than the whisper.  The 7.62x40 will like pmags and such better than GI mags because of the fat ogive of the bullet.  The HRT requires 6.8 mags, but then should be fine.

The HRT wins in the history dept, it uses 30 Herrett data and reamers/dies.  Those are more widely available.  The 30 Herrett data out there is a little conservative, usually designed with handgun in mind.  This makes the HRT a simpler choice, you can use off the shelf stuff and run with it.  

With the 7.62x40, its more experimental, which means performance depends on how you push it.  Depends on what kind of person you are.

Of course .223 brass is far easier to get than 6.8 brass; same for bolts and mags.

The good news is there are so many choices in 30 cal bullets, you can find a good bullet for just about any velocity you shoot at.