I have a bit of a different take on the 'gray area.'

For the longest time, I felt it was just easier to use SubD modeling with polys to get what I wanted. I still prefer it for much more organic shapes which are 'eyeballed' for their form. Things like gloves, or soft goods, or cartoon characters are certainly better in a poly program which supports SubD modeling. Especially if it doesn't need to be 'exact.' This doesn't fit your helmet as it need accuracy, so I think NURBS would be best.

I really like SketchUp for poly modeling even though it's SubD packages aren't nearly as sophisticated as dedicated SubD programs like Modo or Lightwave Modeler or nVil. In particular I like it's ability to create quick interiors with instancing-- something MoI doesn't yet have. It also has the best sketch renderer on the planet.

But now I'm pretty much gonna start every project in MoI, move it to SketchUp for 'sketch renders,' and move it to KeyShot for final renders. It's just terribly faster starting in MoI. Unless it has to be a soft organic, I believe it can be faithfully executed in MoI. This includes vehicles as well.

Just so you understand, when Michael talks about 'surface continuity,' he's talking about a very esoteric subject, and one which is most difficult for all CAD Solid modelers to execute. Rhino needs a third party set of plugins. Alias has a decent implementation- in fact it's fairly intuitive and something I do hope Michael takes a look at implementing some day. For now, a 'good' MoI modeler can use some BLEND tricks with small NETWORKs to get pretty damn close. In fact, if you care to look, there have been some stellar car models created in MoI.

So, the trick is to eventually think in NURBS. A whole different strategy is required for modeling something like the helmet, but it's very possible, and I suspect much easier than you may think. Unfortunately, I'm in the middle of a big project or I'd like to take a shot at it and make a video tute. Maybe someday.

If you think about it, it's a symmetrical object, which has different parts which can be 'joined' or 'booleaned' together with fillets added along the seams. The main helmet should be rather simple to create with a few different surfaces --try taking a sphere, show the points and use the points to resize it. Once you have a decent top half, you can chop it up with different boolean curves.