The general rule that most blade manfg. suggest is three teeth in the work. That means if you are cutting a 1/4' piece of steel you should use a blade with 12th/in for max cutting speed. 12/4=3 1/8" material needs 24 tpi. 24/8=3
Bimetal blades are much better and do last a lot longer than the carbon steel blades; up to 10x or more in hard materials. The bi-metal blade is just what the name implies; two different metals that are welded together with a laser beam. If you look carefully at the blade you will see athin line just behind the teeth where the two are bonded. The teeth portion is a highspeedor cobalt type metal and the large rear section is carbon steel which is more fexible and provides some strength and it is probably cheaper to produce.
There are a lot more things to make the blade perfom better and last longer as well. There are variable tooth blades which have changing tooth patterns thought the length of the blade. For example the blade might change from 14 tpi to 18 tpi every few inches and this keeps the blade from find a non-cutting rhythm and makes it cut better in different materials.
All good manfg. made good and long lasting blades. Look for bi-metal and hi-speed blades from folks you have heard of and beware of cheap economy carbon blades that the one that came on the saw when you bought it. Cheap blades are a very false savings!!!