Both of those canisters are good ones. I went with Rena XP's on my tanks because of the pricing. They work great...nice filters.
On the subject of powerheads...do not think in terms of "how many", but rather in terms of "how much water movement". You should be targeting a minimum of 20x your tank's volume in water movement per hour. With a 40 gallon tank, you want 800
GPH water current in the tank. The powerheads you are looking at will be rated for a GPH rate...buy what you need to acheive the 800 GPH target. You can count the flow from your canister too...so if you get a 250 GPH canister, you only need 550 more from your power heads. When aiming the powerheads, you may need to play around a little to get things right. The idea behind directing one up is to ensure good water flow at the surface, which will help maintain oxygen levels in the tank. If you direct your canister output at the surface, then that will take care of that concern and you can keep your powerheads directed internally. You do need to be sure you have good water movement through your rockwork, as the live rock will be providing the biological filtration for the tank, so direct a
powerhead at the rocks. In both of my tanks, I have powerheads on opposite sides of the tank, pointing in towards the center.
Are you planning on a protein
skimmer? They are pretty important to a salt tank, so I strongly recommend you include one in your plan. I use Coralife Super Skimmers in my tanks, but there are many other good ones out there. The old adage "you get what you pay for" is very true with skimmers, so don't go with the cheapest thing you can find.