1909-1911 T206 Christy Mathewson #307 (Dark Cap) or #308 (Portrait)

[RiceBondsMntna2Young]

Hi everyone, I'm relatively new to these boards and completely new to vintage. I know there's a fountain of knowledge here regarding T206s, so I wanted to ask your advice about the Christy Mathewsons in the set. I've long been fascinated by the mystique and prestige surrounding this all-time great who played for my favorite franchise. I'd like to add one of his cards to my very small collection, mostly consisting of Barry Bonds at the moment. Any insights you can offer as I begin my search would be most appreciated!

Just some things I'm thinking about...

Ideally it should be...
1. Appealing to my eye. Surface condition is underrated and centering overrated when it comes to post-war and modern in my opinion. Is there a (what I would call unholy) fixation on centering for T206s too? Give me vibrant colors, registration, and original gloss any day.
2. A sound investment. Or at least something that keeps up with inflation in case I ever needed to liquidate. It's no fun to watch the value of a card in your PC tank, even if you don't plan on selling it. Unfortunately the market values centering which may be in conflict with what appeals to me visually.
3. Unaltered. I want it to be as free from doctoring and alterations as possible. I'd sacrifice condition and eye-appeal for it.

Dark Cap v. Portrait
I think the #307 Dark Cap is the best looking in the set. To me, it's the depiction that most resembles the photographs I've seen of Mathewson. The more I learn, though, about how much more other collectors seem to covet the #308 Portrait, the more I begin to see the appeal. It's certainly the most striking variation, but it's not a baseball pose, obviously. And I don't think it resembles the man terribly much. But it is a beautiful card. And I'd be lying if I said that how much I wanted a card was independent of how much others wanted it.

Backs
Sweet Caporals seem to be easily the most popular even though (and maybe because?) they are relatively common, with the exception of the 350-460 Factory No.25 variant. Is that right? Are Piedmonts considered far less desirable despite VCP lumping them in with Sweet Caporals? What about Polar Bear, Sovereign, etc. from a collecting, aesthetic, and investment perspective?

Value
Do you think one of 307-309 or a particular back variation is undervalued for some reason? The values for, say, a Portrait Sovereign graded 5, have been relatively steady over the last decade and a half.

Price
I'll probably start looking in the VG-Ex (4) to Ex (5) price range. But I love a good deal and could move two grades in either direction if I found a great price for the card I'm getting.

Grading
Also, I noticed SGC doesn't sell nearly as well as PSA in the same grade. Do you think that's a legitimate or irrational market reaction?

Random thoughts and questions:
1) I've noticed a lot of lower grade cards with perfectly rounded corners as if someone trimmed them to create a more symmetrical silhouette. TPGs don't single these out and downgrade them to Authentic, instead assigning a numerical grade. Why is that?
2) Is there such thing as investment grade in your opinion?
3) How do I look out for cards that have hidden paper loss or filled-in pinholes?
4) What other alterations are common and how do I spot them?
5) How many copies of 307, 308, and 309 are out there do you think?
6) How do you feel about soaking to remove stains? It's maybe the one type of restoration that I'd consider, but I hear even a successful procedure can make the card much bigger from being pressed.

Thanks again!