in my experience,others may vary,Ilford sfx is at best what it claims to be-special effects.it can increase the tonality of a scene and add grain.on very bright days in the late spring,early summer(rich ir times)you can get semi-ir look.during the winter it increases tonality.It's best attribute is it is very forgiving in exposure(up to seven stops,or more)where most near infrared(konica)and infrared(maco)are usually only good for about four stops.with SFX to get any ir effects you need an 89b filter to se a good result(a #25 will generally be a slight increase in tonaltity)with konica(the cheapest of all the 35mm in the U.S.)Igenerally use an 89b to get the effects,although a #29 is good to-but it is a slow film-tripod needed and you need to pick subjects carefully(unforgiving in exposure)or your shadow areas will have no detail,so experiment and see how it is best for you.Maco 820c is my favorite of the group and produces good results,for me with either a 89b(rm72)or a#29 filter,with a #25 I usually get results simular to a regular bw film.It lies in between konica and kodak(also sfx)in grain and contrast,simular lattitude but be careful not to underexpose.
Also each one has a different sensitivity curve and are not sensitive to the same colors in the same amts. so each has it own attributes.I say go out,have fun buy a roll or two of each and see which you like best.
Jim
