My wife and I purchased a pair of Brommies from Portapedal last August. A few weeks after the purchase I was lamenting to the shop owner Al, that I wished we had purchased Superlights. As luck had it, Al had two on order. Done deal. We've had a couple of months with the Superlights now so it's time to share some thoughts.
I believe the weight difference is negligible unless you are a weight weenie, a persnickety, excessive, compulsive cyclist or have to carry the bike a lot. All but the last apply here. I understand that pilots prefer the two speed Superlight due to the additional weight savings. Bear in mind you will need to supply an inflation system as no pump is included. The weight advantage gets slimmer...
So if the weight is not the big deal, why Ti? Ride quality. We live in an area with terrible rural roads, think Paris-Roubaix. On rough surfaces the titanium parts add enough dampening to make a significant improvement, especially on the P bike. While both the M and P bikes benefit, the P bike has more vibration in the bars, especially when you are in the drops. The vast majority of this disappears with a Superlight P, including most of the dreaded hub trigger shifter rattle.
So there you have it. If you are particularly sensitive to vibration and or live in an area with rough roads, the titanium bits on a Superlight may be to your liking. I would not be to concerned about the weight savings unless you carry the bike a lot or plan to fly with the bike in light aircraft.