froponesMr. Pen website says Italic tipped has less pronounced difference between stroke types but gives nib longer life. Then in the next sentence, they say "all our nibs are guaranteed to last the life of the pen of course". The website does not show any writing samples of what tipped italic writes like. I wouldn't bother with it unless you've wrecked an italic tip in the past and are worried about longevity.
Italix also suggests that customers not expect much from ordering fine italic or fine cursive stub nibs as variations in width are less noticeable in fine sizes as compared to mediums. If you want to see true italic magic, start with medium and go for the stub if you want a smoother writer or the full fledged italic if you want dramatic differences and don't mind the occasional angle adjustment while writing.
Finally (inference here), Italix generally offers fine italic choices only on their #5 nibs, with their #6 nibs starting in italic options only in medium size or higher, I would guess then that this is the # 5 nib as used in their Parsons line rather than the #6 used in the Prescriptor and Commission line. That last item is guesswork on my part though.
Take a look here:
http://www.mrpen.co.uk/contents/en-uk/d63.html
The comments on nib selection are (rather annoyingly) interspersed throughout the individual model descriptions.