The BenQ 2050 will produce a picture with about 2-3 times more contrast than the 3500. So in that choice I'd say HT2050 BUT.....
You're looking at the wrong Epson.
Don't look at the 3500-- as Theron said look at the 3100
. The 3100 would be a much
better choice than the 3500. The 3500 is several years old now and the 3100 is a brand new model. It's hard to tell with the model numbers but the 3000/3500 are old, 3100/3700 are new. Epson made significant improvements to the new models including adding features and improving color quality and contrast.
With that said: the 2050 is currently retailing for around $700 while the 3100 is currently $1300 so it's nearly twice the price but it is certainly not twice the picture quality. Both are excellent values for money so it really comes down to budget and what you want in terms of features and installation flexibility. Both have their pros and cons. It should be noted that these two aren't really competitors for one another. The BenQ is more a competitor for the sub $1k projectors like the Epson 2040/2045 while BenQ makes the HT4050 to compete with the Epson 3100. The HT2050 is popular due to it's picture quality where it punches way above it's price class so it tends to be dragged into these comparisons where it maybe doesn't have any business competing. The last thread I participated in had the HT2050 running against an Epson 5040Ub so this is less extreme for sure.
To answer your question: The 3100 will have better black levels and will be overall brighter. The 3100 uses Epson's excellent iris to help improve black levels and dynamic contrast. Keep in mind that the iris movement is not exactly invisible but most don't seem to mind it. In either case most everyone seems to agree it's benefits out weigh it's negatives. Native contrast is actually virtually identical between these projectors. Epson uses 3LCD which tends to produce a very bright and colorful picture but does suffer from the typical LCD characteristics of convergence and motion blur. Epson has implemented CFI on this model to combat the blur and from what I've seen with my hands on time of the 3700 the convergence is much better than on prior models. The really big deal about the 3100 is it's competitive picture quality combined with it's placement flexibility. If you are setting up a projector for the first time I cannot overstate how nice it is to have generous zoom as well as vertical AND horizontal lens shift. This projector is very easy to fit into any room.
The HT2050, as I stated isn't really in the same category as the 3100. As such it has MUCH less placement flexibility. You get a little bit of zoom and a modicum of lens shift-- vertical only. I hesitate to knock it for this though as in it's price class this is way more than most of it's competitors offer. Still, you might find you're fitting the room to the projector due to the limitations. On the plus side the BenQ has a shorter throw than the 3100 so it can project a larger image without needing as much distance to the screen. The HT2050 is a single chip DLP. It is bright but isn't as bright as the 3100. Both will satisfy in a mixed use room but the 3100 will likely have an advantage in ambient light. Typical to DLP, native contrast is good and the projected image is VERY sharp. DLP typically has an edge in sharpness due to it's single chip design and high pixel fill. Keep in mind that single chip DLP can produce an effect call RBE or Rainbow effect. This is an issue where the viewer is able to perceive the sequential color creation of the DLP light engine as a rainbow trailing high contrast fast moving objects. The BenQ's use of a high speed RGBRGB color wheel mitigates this for the vast majority of users but your mileage may vary. Conversely, the BenQ has superior motion resolution and does not blur. The HT2050 has no CFI feature though so if you're into that you'll need to look elsewhere.