YMMV, but I've found the 40's Super Speeds to be a very nice balance of smoothness and aggressiveness. Not too much of either. Load one up with a sharo blade such as a Feather or Astra SP and treat your face to a great shave. I've found the flare-tips to be too mild for my face...they require too many passes to get rid of the whiskers, and too many passes can result in irritation.
My experience is a bit different than Snargle's. I find the two types of razor to nearly indistinguishable in performance. The only obvious difference is the feel of the handle. Full disclosure: I have 3 40s style made between 1953 and 1954, and one flare-tip made in 1954. So they're all made within a roughly 1.5 year span. Not sure if that makes a difference.
I have one notched no date code, one 1953, a '56 flare-tip and '58 TV for comparison. As best I can tell the shave is identical with all, coming down to which handle feels better. For me, I'm undecided - kinda like both styles in their way.
I'll add to the diversity of opinions: the original '40s Super Speed is a much better shaver for me. It's both smoother and more effective than the Flare Tip.
IMOHO Super Speeds seem to get milder the newer they are.
I've got a NDC early quarter 1950,a 1953,a 1954,a 1955,a 1958 Red Tip and Grandpa's 1970.
The 50 and 53 are on a par with the 58,the 54 and 55 are nice mid range shavers (ended up PIFing the 54 onto a noob wet shaver) but Gramp's 70 see's little use as it just feels way too tame for me.
Personally, I find more difference between a 50's style flare tip super speed and a 50's style flare tip super speed. Yes, you heard me correctly.... and I'm not going nuts. I think that the pre-1957 flare tips (the ones with the smaller opening in the bottom of the TTO knob and the screw) feel and shave more different than the flare tips made 1957 and later. I think 1957 is the year they changed them, maybe 1958.... someone correct me if I'm off. To me, the pre-57 flare tips are pretty similar to the 40's style.... but they both differ from the post-57 flare tips. The older super speeds seem a bit more aggressive and solid in my hand. YMMV.