I have the Boye, and you are correct that the cable thickness/stiffness is comparable to the regular needles. The only advantage there is that you generally store the cables flat, so you don't have to dip them in boiling water before using them!
The tips of the Boye interchangeables seems sharper (in a good way, IMO) than my regular Boyes, however, my regular Boyes were handed down from my mom - she got them in the 70s. I can't say how much she used them though. She tends to knit like crazy for a year or two, then not knit again for 5-10 years.
One of my size 9 tips unscrewed the first time I used them, and it bent the rim a little, which makes it catch a little (I have read here that I can file this down, but it hasn't bothered me enough to motivate me to try). Since then, I am OCD about checking, checking, checking to make sure they are properly tightened. Occasionally when I check, I will find one needs to be re-tightened, which makes me nervous enough to continue to obsessively check.
I would love to get the Options to compare, but can't justify having two sets at the moment (I am spending myself out of house and home on Malabrigo!), and since you are allergic, sounds like those are out for you.
I am not sure how useful I would find the needles w/o the whole set. In your position, I might try to save up to buy the kit all at once at a later time (maybe with a JoAnn 50% off coupon? They come out about once a month, or so I am told).