Are you generally more careful, or carefree with your bags? What's more important to you, fuss free usage, or getting the bag that your heart beats most for? If you're someone who doesn't baby her bag at all and wants to enjoy her bag carefree with the bag showing as few signs of wear and tear possible, I'd go for calfskin. It's definitely more hardwearing than lambskin.
I have a black lambskin old medium boy and I chose that, because I love the luster and classic look of lambskin. I also very much like calfskin and might have chosen for that one, if only back then they had the style and color combination that I wanted.
I have the lambskin boy for about 2 years now. I use it often and although I'd love to tell you that I'm not at all gentle on my boy, use it completely carefree, bump with it into things without a second thought, put it on the ground everywhere and it still looks in mint condition............but I can't.
That's just not the nature of lambskin. Lambskin is as soft as leather can come, looks and feels divine, but does come with a price. Although many are of the opinion that lambkin is more hardwearing than it seems (and I somewhat agree), but fact is still that it's a fragile leather and requires more care than other leather types to keep it looking scuff and scratch free. Minor and surface scratches can be buffed out, but with deeper scratches and scuffs (have had that happen on one of my other lambskin flaps) your hands are pretty much tied. You'd have to send it in for professional refurbishment and even after that, it won't look and feel the way it used to.
Condition is also very subjective and each person has their own opinion on this. What's "excellent" to one might only be "good" to another. I admit that I'm pretty anal about my bags and see every dent and scratch (especially if they are lambskin cross body bags and cost an arm and a leg, lol). I enjoy my bags, but at the same time want to keep them in pristine condition as long as possible.
With that said, in my eyes my boy is still in very good to excellent condition after many uses, but that wasn't achieved without blood, sweat and tears .........hahaha, jk.
Because I really wanted this boy I don't mind at all the extra care that it requires (mindful of my surroundings, don't take it to super crowded places, not bumping into people and things, never put it on the ground, condition it with leather lotion, buffing out surface scratches as I see them, store it correctly and not growing talon nails). It's a give and take, IMO. On days that I want a truly carefree bag, I'd pick my caviar, patent, or calfskin ones.
Anyhow, I don't want you going in having super high expectations of the durability of Chanel lambskin (it's not the way it used to be), only to be disappointed (especially since this is your 1st Chanel). Buying the lambskin boy was worth it for me, but whether or not it will be a worthwhile buy for you depends on how you are with your bags and whether you mind some scratches/scuffs here and there and like the "broken in" look, or prefer to keep your bag looking new with as little care as possible.
It's a very expensive luxury purchase after all, so give it much thought and remember to come back here to do a reveal