For leaders, in the smaller pound tests, I don't think there is much strength difference between back to back nail, uni, or blood knots. I start off with back to back nails, and use bloods when tying on new tippet when fishing for non-big game fish. Bloods can be tied with a minimum amount of line in the field, so you don't shorten up the section adjacent to the tippet by much. But I clench them up underwater because, of the three, they are easiest to "burn" or deform when clenched up.
I now use the loops the manufacturers put on the lines for connecting leader to line and backing, since I have heard good things about their strength in the last few years. Also because I have a stock of discontinued "new in the box" solid, hard mono-core tropical floating lines.
Nail knots are not particularly good when tied to solid mono-core, as they tend to compress the coating too much at the end of the knot and can pull it off, leaving a little piece of mono sticking out the end. One of those lines had the factory loop looking pretty suspect after a while and I replaced it with a mono-braided loop.
For fresh water fish , I would just use nails for both leader-line and line to backing. For leader to leader, bloods.
PS: An easy way to tie nail knots is to bend a thin piece of leader wire in half and squeeze to a good point. Then wrap around it and the fly line with the leader/backing, then shove the wraps tightly together with your thumb or finger nails. Now stick the tag end into the wire loop, and pull it under the wraps while spinning the line and wraps between the thumb and forefinger of your other hand. You will never get cross-wraps this way and the wire loop can be opened way up to make sticking the tag end into it a piece of cake.
Snug the knot a bit and get the wraps all touching each other, then slide it down near the end of your fly line before cinching up all the way.
Cheers,
Jim
