Is the difference strictly the design?

Yes, that's pretty much the only difference.

Yes, it's pretty much the design and nib selection. The 1911 is larger than the Professional Gear, and it has tapered ends while the PG has squared off ends. The 1911 has a greater nib selection--you can get pretty much any Sailor nib with it. The PG has a subset--not sure exactly, but you can visit the Sailor website for more details.

The 1911 comes in two sizes: the Large, fitted with 21K nibs, and the Standard, fitted with 14K. The chart (above) only shows the Standard.

I'd suggest trying the pens before purchase if you can. I find the Professional Gear a bit short compared to the 1911 Large, but then I have a large hand. Both pens have many fans here.

The Professional Gear body and the 1911 body are made for the same parts except the end caps. The cap and barrel ends are flattened on the PG and rounded on the 1911. Also, the PG cap top features an inset Sailor logo. If compared from the top trim ring to the bottom trim ring, the pens are identical. It's just at the ends that they differ. The same applies for the Sapporo/Slim and the smaller sized 1911. There's no comparable 1911 size to the PG Mini pens...

I just took a look at the pen measurement chart at nibs.com. This is a great resource. Scroll down to Sailor for measurements of the different models.

The 1911 Large and Pro Gear are not really different pens.

The caps and barrels are the same, the only difference are the ends (they're flat on the PG and curved on the 1911). This is what leads to the size difference.

Also worth noting is that the 1911 come with solid colour standard nibs whereas the PG comes with dual-colour nibs.

If you look at this (slightly wonky) photo you can see that the sizes are the same.

I have 1911s and a Pro Gear. My favorite color is orange, so the new PG had me reaching for my wallet within seconds of the Sailor rep. opening the box at the show.

At first, I was a little concerned it wouldn't suit my hand as well as the 1911. Not so. I've now used my Pro Gear for extended writing sessions and I love it.

The photos and the numbers above speak to and seem to confirm my experience with these pens - the difference is primarily in the aesthetics. Both are quality pens and, to me, handsome designs.

The M nibs I have in these models look different but write identically.

I hate to hedge on this but, as with all of these things, YMMV. If you're interested in the Pro Gear, and you can, try one out in person.