I use the dip rack and the handles put me at a slight forward lean when I'm doing them. For me the slight forward lean is necessary to get the full range of motion. This is because I have tight shoulders. If you are very flexible then you could probably do them the other way but I would reccomend starting out the way I do it.

The reason you are finding it hard to add enough weight to your bench dips is because you are effectively using roughly half of your body weight and half of the weight added as soon as you put your feet up on something. By doing them in a dip rack you will be using your entire bodyweight plus the weight added.

Hope this helped.

IW