Spools vs. Reels...
Spools are very simple things, but need special techniques to use, particularly in 'knitting' the double-ender to keep it (otherwise it will escape). Spools have one really irritating drawback: you drop them and they unwind all the way to the bottom. They're slightly fiddly on the surface to clip together (I use both the double-ender
and the SMB's bolt snap). Spools need some practice.
Reels come in small and large. Small's nice to fit in your pocket, but they're fiddly to wind. Larger spools are nicer to wind, but won't fit in your pocket.
If a reel's kept outside of a pocket, you must ensure that the bag can't escape and unravel. Similarly with ensuring that the reel remains locked (I use bungees for that).
For single-cylinder recreational diving, 30m/100' to 40m/140' of string is plenty long enough. If you're doing deeper technical dives, longer string is very helpful especially if you release the SMB from the bottom in a current (so that the boat can see your SMB before you drift substantially downstream in the tide).
An excellent starter reel is something like these MDE reel and Beaver reels. Lightweight and 'large' with bungees to safely retain your SMB. Don't use the 'suicide' clip on the end of the string; just use a 40cm/15inch long loop (big enough to fit the spool through; pass loop through "hole" in SMB and back over the reel; instant knot).
I use two reels, my backup is kept in my drysuit pocket and is a mini pocket sized Custom Divers reel on a Halcyon SMB. This reel is basically spool sized, so fits easily in my pocket on a bungee loop -- hence the SMB is rolled "inside out" so to keep the size down (with the nozzle pointing down into the pocket so it doesn't catch when pulling it out).
My main reel is my primary Kent Tooling reel with 100m/330' of line on a CO2 cartridge inflated SMB, this is clipped off to my waist D-ring or on my rebreather. This is a much larger reel and a very substantial metal ratchet mechanism. Larger reels are much easier to wind in when on the fast ascent (10m/33' per minute) portion of the dive, before arriving at the decompression stops. The weight of the reel helps stabilise the string in waves/swell.