Testor's red is the same as Bondo red spot & glazing and Dr. Microtools and 3M Acryl red; good for minor dings and scratches, can be sanded within 45 minutes if the layer is very thin. Like most lacquer based putties it will shrink some and badly so in thick sections or under paints with strong solvents.
3m Acryl blue, probably the best lacquer based putty for feathering and least shrinkage. One of two lacquer based putties that scribes decently.Squadron white, OK for basic filling, terrible shrinkage, not too strong, bad under paint unless heavily primed or sealed with thin CA. Doesn't scribe worth crap.
Tamiya lacquer based putty (silver grey), good, sands well, string, shrinks more than Acryl blue but not as bad as others. Second that scribes decently
CA.....only if I have to and I sand it as soon as it sets, accelerator makes t sand easier. Plasti-Zap more sandable for an hour or so than other types. I do not like microballoons. Scribes very well if difficultly.
Milliput, Apoxie Sculpt or Hexcel A&B putty......only if I don't need to sand much; great for base filling large areas then use lacquer or polyester on top for sanding to final shape.
Polyester putty, AKA Bondo two party stinky stuff.....also made by Tamiya.....both work, both stink like you wouldn't believe, the Tamiya has a slightly finer grain and feathers a bit better. I find this and the red or blue lacquer based putties best on resin since they both stick very well to it and styrene. Scribes very well with sharp hook type tools, forget using needles.
For most simple gaps I usually glue a shim of sheet or strip styrene in place (with CA for resin parts or liquid cement for styrene parts), let dry well, trim with a scalpel and sand. Strongest and best looking seam you'll get. Best scribing filler.
For filling large holes and such, don't discount rough carving a chunk of balsa, sealing with several coats of CA, gluing in place then touching up with lacquer based putty.
Vallejo "plastic filler" works well on figures but doesn't sand worth a damn! Like wise white glue works for some gaps but does shrink and again doesn't sand even remotely well.
The best way to avoid filling is to dry fit and modify parts beforehand. Sometimes it just takes altering how you assemble, like gluing part of the wing in place and once that's dry gluing the other part in place and using clamps or your fingers to hold things in the right place until the glue is set enough to hold and eliminate the gap that way.