Are you looking for an engine, or chassis?
Both engines are very good and reliable. The preference of DI vs IDI will dretermine the electronics involved. If you are considering the engine for a project non MB chassis and are leaning towards a 648, you may want to also consider a 642 as the more compact packaging opens up more transplant options.

All 3 engine offerings have their pros and cons. From my limited experience, the DI engines will yield more power for the buck than the IDI ones, but the IDI ones will likely be more user friendly and reliable sans the electronics. So it will depend heavily on what your primary objectives are. For me personally, I found balance in having both. My 642 is my power engine, my 603 is my reliable engine. Even though both are 3L sixes, both are very different.