While there's lots of discussion about vintage Parkers (especially the 51) in the NIB, there is precious little about vintage Sheaffers.
Leaving aside the whole Sheaffer vs. Parker issue (you'll often find that folks have a preference for one or the other), I think folks just getting into vintage FPs should think about trying one of the Sheaffers with the Triumph nib.
The Triumph nib was a radical departer from the traditional "open" nib in that it consted of a nib-shaped piece of metal that was rolled into a conical/circular shape and had the seam welded - effectively creating a collar that was then seated to the section. The effect was a very strong nib that could take abuse and be used to make carbon copies. In addition, the nib was slightly upturned and could be used upside down when a very fine line was desired.
As they say, a picture is worth 1,000 words (this was taken from the Penhero site).
In addition to being a very strong, functional nib, in my view, it is also a very attractive nib.
I recently acquired two Sheaffers with the Triumph nib - a balance with a medium nib and a tuckaway with a fine nib. In addition to really liking the look of the pens, I have been completely blown away by the nibs. While the nibs are very stiff (as was intended) this is no different than the nibs on most 51s - remember, these pens were designed to withstand everyday use/abuse. This makes them as good an every day pen now as they were back in the day. Also, the durability of these nibs means that you can often find Triumph-nibbed pens in the wild that write great (same as you can often find 51s with nibs in great shape because they also have very strong nibs).
Now, the one downside to the Triumph nibs is that they are typically found on Sheaffers with the vac-fill or touchdown fill system - both of these are are more complex to service than either the Parker vac-fill or the aerometric (which is as bullet proof as you can get) and the vac-fill system is somewhat delicate due to its long thin plunger rod. That being said, when treated with the proper care, the Sheaffer systems are quite reliable, but if anything goes wrong or you find one in the wild that has a busted filler, you'll likely have to send it out for repair - which will typically run you $40 to $50 (vs. $30 to $35 for other types of vintage fill systems).
That being said, there seems to be less craziness over the price of these pens than there is over Parker 51s and good, restored examples can be had for well under $50.
From an ergonomics perspective, while the tuckaway (as its name suggests) is a small pen, the balance has very similar dimensions to the Parker 51, which I think says something about the right size of a pen for everyday use.
Overall, I give the Sheffer pens with the Trumph nibs a big thumbs up and think they're well worth seeking out if you are just getting into vintage FPs or want to add what I consider to be an iconic pen/nib that represents the best of the heyday of fountain pens to your collection.
For a much more in-depth article see the great one at Penhero.
Thanks for reading and happy scribbling!