having recently acquired the awesome omega d5v XL I am in need of an enlarger lens for 4x5 negs. What should I take into consideration and why should I pick a 135 mm over a 150 mm lens, or the other way around? are there lenses I need to stay away from?
I would go with the 150mm. I have never been able to get a 135 to cover a 4x5 properly. Others with far more experience then I will beable to help you here.
well, that seems to be unanimous.. I'll look for a 150, should I prefer a 4.5 over a 5.6 ? I don't know anything about these lenses, is there a test or website I could use?
Both the original 135mm lens on my ancient DeJUR 4x5 enlarger and a somewhat later f/5.6 135mm El-Nikkor cover 4x5 negatives well enough. However, if you have high enough darkroom ceilings and long arms, a 150mm might be a better choice. The main advantage of a maximum aperture of f/4.5 is easier focusing without a focusing aid. Any modern high quality f/4.5 or f/5.6 lens should provide fine sharpness and contrast. Even my older lenses suffice for me.
The 150mm is the best bet. The rule of thumb is to use the longest lens you can get away with. I have an old 135mm EL-Nikkor that seems to work quite well, but I have heard that many other 135 mm lenses suffer a lot at the edges.
And if you have shallow and narrow pockets, go for the Tominon 135mm taken from one of the Land copy cameras. I took one from loading dock trash pile and compared against Componon and I was surprised how good it was. It covered 4x5 with good margin on all sides and gave good image to the corners. BUT you might want to keep it quiet that you use Tominon on one of these geeky forums... Frankly, I kept my Tominon and sold my Componon. (I sill keep Componon 50, 80, 105, 210mm...)