If your horse is used to a snaffle, don't go to a port. An argentine works 100% differently than a tom thumb due to the bend in the shank. I never use solid mouthpieces on a horse learning to neck rein, I find they simply don't work as welll as argentines.
If she does eventually switch to a ported bit, it is best to be LOW ported, because the port depth/height will add severity.
If she is going switch bits and ten teaching to neck rein would be incredibly confusing for the horse and take almost twice as long. I thought she was going to train on the snaffle and then change bits. It doesn't work all to differently because it still has a lot of similar design including both shanks, and a broken mouthpiece.
Argentine snaffle or a Tom Thumb are a lot alike, more than you'd think. Tthose bits are more severe than a similar solid mouthpiece curb because of the nutcracker effect they can have on the mouth when you put any pressure on the reins at all. I would rather use a short shanked grazing curb bit, or even a regular snaffle as we already mentioned for training a horse/continuing training with a horse.
Edit- I just wanted point at the that the bend in the bit REDUCES nutcracker effect but does not eliminate it. You would have to have a third section in the mouth piece to get rid of the nutcracker.
Ok, I finally found something similar to what I train in.
Here it is:
It has four joints in it: two where the bit connects to the shank on each side and two breaks at the base of the port. If you don't use the bit for steering very much this bit works great. It's very flexible and it will communicate your intentions much better because of it. But DressageIt has a good point. If you're not the softest handed rider, you might want to work on that before you use this. Normal cues that you give through the bit are amplified by the port, so you need much less pull for routine commands.
And if your going to show in a solid bit, like I do, you should use the solid bit every now and then to keep the horse aquainted with it.
Quixotesoxs , that's kind of what I was leaning towards. I like the double-joined mouthpiece (my boy had issues with his single-jointed snaffle - nutcracker). I just wanted to be sure to avoid the bad communication effects of something like a Tom Thumb.
I started a teensie bit of neck reining today and he seemed to be catching on. Well, he may have just been listening to my leg aids, but I included the rein pressure as well.
Hmm, just a random comment...... Argentines are normally for horses who already have developed good mouths.... that way the nutcracker effect is more avoided.... we never put a horse in an Argentine enless their ready..... but then some of the things my trainer does I personally don't like, but I'm always open to new ideas..