How much accuracy difference can I expect from reloaded neck sized fire formed brass verses new full length sized brass? I've reloaded most of my .223 cases 6-10 times and am thinking I need to buy a bag of new. Thanks for your help.

The only way to tell is try them. I doubt in a factory rifle or under less then ideal conditions you`ll be able to tell the difference. The neck tension will be starting to vary some from work hardening but if they aren`t splitting there is no reason to discard them. You could anneal them if you have the desire and probibly get another dozen loads from them.
I discard my brass at 10-12 loads but I`m full sizeing and at that point I`ve trimmed them 3-4Xs and feel they are thin enough to warrent new brass, not because of any noticed problems.

22 HORNET: You ask a great question, that I am confronted with sometimes several times a year! My question (worry) is similar to yours and I will restate mine here as well as my answer.
I have one real obsession in life and that is Varmint Rifles. I have bought at least 7 in the last 12 months. That is a little above the norm for me for any given 12 month period. Ergo the seat of my problem! I have lots of Rifles to shoot and I do not want to (unless absolutely necessary!) fireform all my brass that will be dedicated to a particular Rifle. What I am saying is I always work up an accurate peppy load for my new Rifles by first of course shooting the virgin brass and then reusing that small batch of testing brass to reshoot and retest for loading accuracy. So I shoot the new brass and usually some time later after maybe 3 or 4 firings I settle on my load for that Rifle. Now what to do about the majority of my brass for that Rifle that is still virgin and will be full length sized for its initial loading. Well over the years, actually decades now, I have found that my chosen load almost always will shoot very nearly as well as the load developed in the fireformed and neck sized brass.
So my answer to your question is that in my experience it won't make much difference in accuracy at all. I go ahead and shoot the virgin brass reloads in the fields (instead of fireforming at the range!) at Varmints with good results. I then later integrate my test brass back into the main group at the proper time (number of firings).
6 reloadings I would keep with them. 10 reloadings is nearing the useful (safe) life of good peppy 223 loads brass. I would replace these 10 times shot brass. Brass is cheap especially for the 223! I keep close records on my number of times fired per lot of brass.
Hope this kind of vague answer will help you some. I ponder over it like I say several times each year.
As a test to answer for yourself and for this particular Rifle go out to the range on a good calm day and shoot a couple of groups with the old prepped brass and then do the same on a calm day with the new brass! Compare the results for the two.
Good luck.
Hold into the wind
VarmintGuy

Thank you both for your input. I definately feel a lot better now knowing the accuracy "should" be minimal. I'll try at the range as soon as I can and let you all know. Thanks again