Hi,
The 136 vs 159 for camper conversions gets debated a lot here -- with strong proponents for both.
We have a 136 conversion for the two of us, and it works fine for us -- just got back from a month long trip and we are still talking to each other
The 136 parks and drives and U-turns just like a car. Its a bit wider than a regular SUV and that can cause us to pass up a narrow parking space once in a while, but generally not an issue. I've gotten so I can parallel park the 136 PM easier than I can our Honda Pilot.
I'd test drive both lengths and try parking with both.
When doing a conversion, the 136 allows enough room for normal living for two, but not enough room to easily include a shower -- so, if shower is important, I'd go for the 159. Also if you are anticipating living in the van full time, I'd go for the 159 just for extra storage space.
A good step would be to look at floor layouts for your conversion -- there is a thread here that lists a bunch of them:
http://www.promasterforum.com/forum/showthread.php?t=53962&highlight=floorplans
More on floorplans here:
http://www.buildagreenrv.com/design-and-build-information-for-camper-vans/choosing-a-floor-plan/
You can go a step further and do a rough real mockup of a floorplan you like and "live" in it for a while. This can be as simple as tape on the floor, or some simple cardboard mockups of the beds, galley, ...
Its worth spending some time to make sure you get the best length for you.
Gary