Would you pay for preferred or MCE or neither?
 

Going from San Diego to London with a change of planes at DFW. On the first SAN-DFW leg, GF and I had decent standard seats in row 21, but someone took the middle seat, which surprised me, as there are a good number of aisle and window seats available from row 27 and back. I think the plane is an A321, but none of the charts on SeatGuru seem exactly correct.

On the first leg, which is about a 3 hour flight, would you?

A. Stand pat even with someone likely in the middle seat.

B. Spend about $36 total for a preferred window and aisle seat in row 15. They charge in the row for the middle seat, but not in other preferred seats, making it less likely that anyone will sit there.

C. Spend about $84 total for MCE, probably row 9 one row back from the bulkhead. Row 8 is available at a slightly higher cost, but seats are more narrow and not thrilled with the screen in the armrest.

I wouldn't make any decisions based on an empty middle seat. If nobody buys it, chances are very good that somebody without a seat assignment will get it assigned to them on day of travel.

Never count on a middle seat remaining open. However, whomever gets stuck with it will likely be happy to switch with one of you so you can sit together (with one of you in the middle).

Best A321 layouts are on Aerolopa now that may help you better choose where to sit. I suggest you do the same for your DFW-LHR (even if in a standard seat):
A321 OASIS (i.e. not an A321NEO or A321T): https://www.aerolopa.com/aa-airbus-a321k
A321NEO: https://www.aerolopa.com/aa-airbus-a321nx

I'm confused...you are going to London and spending $84 is some kind of issue?

Regardless of whether you're going to London or Lubbock it's a perfectly reasonable question to ask whether spending money on MCE on X plane or X route is worth it.

Like most here I travel all over the world but that doesn't mean I just want to throw $84 away if I can avoid it.

The A321s have a few MCE rows with really fantastic leg room. If I didn't have status, I would consider paying $84 for 2 in MCE, but others--with less disposable income might not.

Nope. I'd go for the last row if I could for free.
(just because I think the chances of it being more empty are better. It seems rare to me that MCE isn't filled up, but I don't know this London route...)

We're actually going to Paris, although the final leg is on BA and didn't seem worth mentioning.

The cost isn't a huge deal, but just strategizing. I didn't really consider that the middle seat might get filled within 24 hours anyway. And the SAN-DFW leg is pretty short, at only 3 hours or so.

It's also worth mentioning that if your flight is still a ways out, there's a pretty good chance that a schedule change and/or equipment swap would change your seating calculus.

I actually like row 8 bulkhead seats.Plenty of legroom and you can still store a bag underneath the seat in front of you, which avoids the normal bulkhead hassle during take-off/landing. While the seats are a bit narrower, I also like the fact that they basically “enforce” separation from your seat mate. This I appreciate even more after a recent flight where the person next to me basically spilled into my seat both above and below the armrest.

Not sure I understand the comment about storing a bag under the seat in front of row 8. Is it a curtain or a solid wall? If a wall, then I assume I wouldn't have easy access to storing anything under row 7.

How tall are you? MCE or exit row is almost necessary for me for a longer flight as it becomes tough to get comfortable as my legs can't really move. For my gf, she's a foot shorter, so less of an issue.

I'm about 6' 200 lbs. GF is about 5' 1" maybe 115 lbs, so not an issue for her.

It’s not a wall. It’s just a flap that hangs down a bit past the top of row 7 so you have full access to the space under row 7.

FWIW, I just upgraded to MCE but row 9 rather than bulkhead row 8. It was slightly cheaper ($81 total), and I like the extra seat width. Plus, I prefer easy under seat storage and I'm not a huge fan of the video screens in the armrests.