I'd say it depends on what your preference is/what the split of your work is/what programs you use. Most of the benefit of workstation graphics cards is in 2 parts. The first is that the drivers (and possibly hardware), have been specifically tweaked/optimized for programs like CAD/3D modelling etc etc. The second part is ISV, which is linked to the tweaked drivers and what not, it basically says that the graphics card has been certified for various programs, and gives you support in case you ever have any issues.
According to NBC,
NVIDIA Quadro K1100M - NotebookCheck.net Tech the K1100M looks to be based on the 750M. Same architecture (G107), same number of CUDA cores, no clue on clockspeed though. So that being said, they should be roughly equivalent in games (The K1100m in the M3800 vs the 750M in the XPS15). I'd say the 750M will probably be slightly faster in games, since it has the driver tweaks (so the K1100M will be optimised for productivity apps, and the 750M will be optimised for games).
That being said, it largely depends on the apps you're using (and to what extend ISV is important to you, ie, some engineering firms require ISV certification etc). I'm also an engineer, and I just finished working on a project at home on my 4930K and GTX680, ran Autodesk Inventor like a dream (it seemed to be more CPU limited, it barely touched the GPU). If you're using apps that are optimised for CUDA, then it'll make a difference, otherwise either should be fine.
Finally, like you pointed out, it depends on the price difference.
I went for the XPS15 because i'll probably never have to do any engineering work on my laptop asides from running excel and the occasional CAD drawing (i'll be be using a work computer for engineering work).... sooo yea.
Problems? See this thread at archive.org.
