Normally, if you asked me how to best spend money in a Free-2-Play MMO, I would tell you to buy a subscription. In most F2P MMOs I've seen, a subscription basically replaces the F2P structure. You subscribe, and everything unlocks. Most subscriptions even give you some F2P currency so you can indulge in some of the cosmetic items for sale. The idea here is that you can either go a la carte, and pick up exactly what you want in bits or pieces, or go for the one price which gets you everything.
In Blade and Soul, though, I'd tell you hold off on the subscription, or Premium, option. First, BnS Premium can't be purchased directly, you have to spend the F2P currency, NCoins, to buy it. Thus it doesn't come with a NCoin grant, because buying it for 2000 NCoins and getting 500 back would be silly.
Second, Premium doesn't actually unlock anything other than the wardrobe. Admittedly the Wardrobe, which stores costumes, is nice. Premium grants you a lot of bonuses: increased loot from mobs, increased XP, more spins on this Daily Dash login game, etc. But you still have to purchase extra character slots and inventory expansions separately.
I'm not saying not to spend money on BnS if you want to. Just don't fall into the trap of thinking that you can just buy Premium and you can ignore the cash shop, the way you can in SWTOR, for example.
Rather, if you're spending $15/month, or whatever your budget is, you should put your first purchases toward inventory expansion items, additional character slots, multiple skill specialization slots, etc. Unlock the full structure of the game first. Then in the second or third month or whenever you're satisfied with all the unlocks, you can purchase Premium if you want.
I guess this post is prompted by a bit of buyer's remorse. I picked up a month of Premium, simply because I'm in the habit of assuming that subscriptions are the best way to play this genre. I really should have spent that money on unlocks instead.
Still, it was an interesting look at a setup where the subscription does not replace the F2P elements, but just acts as an extra boost on your characters.