Douglas fir
Warm reddish tone, straight grain, holds a stain evenly. Reads more traditional. Our default for coffered ceilings and vaulted great rooms.
Knotty alder
Softer tone, visible knots, more character per foot. Reads more rustic. A good fit for cabins and Southwest ranch homes.
How to pick
Match the trim you already have. If your baseboard is fir, your beams probably should be too. If your cabinets are alder, alder beams tie the room together.