Yes, Harold, the more figured a grain is, the better the chances of the wood tweaking.
Douglas fir is considered a softwood, Iya. In fact it is most often used in the framing and siding of houses. It's grain can be very straight and on that basis is really strong in the direction of the grain, under compression, but it's grain is also very porous, and so it soaks up water. It's used in boats as stringers and such as it doesn't have a lot of oil in it and the resin will soak into the wood a bit and increase the bond of the fiberglass. If you can keep it dry, it's a good, light wood that will take a grat deal of weight on it's ends.
Ironwood is indeed some pretty fun stuff and heavier than Oprah, but it's real resistant to worms and rot. A similar wood, Apitong exhibits the same features in a slightly lighter wood and also has some very straight grain.
Teak is just so damn cool in that if you keep it oiled, or submerged in salt water, it'll last for generations. It will rot with freshwater exposure and the subsequent drying out, and so the bond between it and the deck or the stringers needs to be very closely watched. Dolfinite or 5200 and a good fastener schedule will ensure that the stuff will reflect your pride of ownership for years.