I suppose its about deciding the purpose you required? I got the 28mm and 50mm, in my opinion, both have different uses though they are useful under low light. It you want to combine both landscape and potrait, pherhaps the Tamron 17-50 2.8 covers all your needs. You do not have to change lens and risk dust or damaging your lens too. Both lens prices at close to $500, which is the price of the Tamron. Shall you decide to get the prime, you gain in aperture but lose on the focusing distance.
Know your requirement and determine your budget and then enquire the marketplace sellers for a price. After making your decision, then fixed your deal to purchase your lens.
I encounter someone here before who started asking for price, bargain the price, reserved the lens, and then disappear without being responsible enough to even inform that he changes his mind. =) kind of insincere person to help in the first place.