For digital lens focal 28mm = film type focal length 40mm
For digital lens focal 35mm = film type focal length 50mm
Are the above estimate correct ? So for prime lens will the 35mm be better as it is near the film type focal length of 50mm.
Don't be confused between focal length and FOV. Focal will never change on the lens regardless if the sensor is full or cropped. It's the FOV that changes. This terminology MUST be handled very carefully if you want to further understand the effect relative to DOF.
There is NO BETTER. Some people don't like to get near to their subject, using 50mm for DX will get the FOV near to 75mm which is good for them. Some people like to follow the FOV of a 35mm film, and hence use 32-35mm focal length on cropped sensors.
Go to the shop with your own camera body and look through the viewfinder and see how relax or tight the FOV is for each lens focal length. It will be a mistake thinking popular is what you want.
Me for example, like to work with wide angles regardless on DX or FX, and I don't mind moving up close to the subject, like in one of the marathon event. I'm basically right infront of them at 24mm on FX. The kind of perspective is very different from standing far away. Perspective distortion at each focal length gives a flavour and mood to the kind of photo you are taking.
If you are not sure, I recommend you work with a general zoom lens of focal length between 17-55 for DX and 24-70 for FX and see which you use more. What's the point of going for primes if you don't even know which focal length you will use the most often ?
Going for popular or recommended is silly, because different photographers have different shooting habit and no one can speak of how you should aim and shoot.