Loop to Loop is the only way to go.
Nail knots are a pain, make your life easier. If the fly line you bought doesn't have a loop, it broke off cause you've been fishing hard (happens to all of us) or you're building a Frankenstein shooting head, a few packs of these will always put in good stead. Easy to apply, smooth to roll through guides and easy to change leaders or sink tips. In two sizes to fit most fly lines, these are really a no brainer.