Angie's the expert, but I'd say to go with the petite, if you often have trouble with regulars being too long in the armholes. I say this as a petite 5'0 tall person. I have a 5'4" in daughter who's on the cusp between regular and petite -- she is petite on her upper half, regular on her lower half, and yes she has a long torso.
In my experience, it's far easier to raise a hemline than it is to raise an armhole. It seems like it'd be easy enough to nip a few inches in at the shoulder seams, but it's not always (and anyway doing it raises the waist).
The question with the dress you've chosen is where will it hit on your waist? Your torso may be long, but where does your waist land? For example my 5'4" daughter has an exceptionally long torso, but the skinniest part of her waist lands fairly high, so when dresses or jackets have a defined waist, petite generally work best. But if it's a top and waist is not defined, then regular works best because she needs the length.
When it comes to ordering online, if you have doubts, email the web site. I have had great luck asking for detailed measurements. It may take a day or two, but I always get a response. Go measure a similar dress in your closet that fits well. Ask for measurements from shoulder seam to waist. Ask for length from shoulder to armhole too.
Don't worry about length from waist to hemline -- that is the simplest and cheapest alteration to make. Your dry cleaner can probably do that.
Or, just order both to compare -- and plan to return one.
P.s. It's a lovely dress!
