I'm a fan. I agree that you're signing up for a shorter time with an older pup but Poptop doesn't seem to be scared off by that. You do know what you're getting into though and it is a consideration. Our first dogs were boxer siblings, they died 2 weeks apart. Our second dogs were bullmastiffs who died one year apart. We just are not guaranteed even another day with any of our dogs and their lifespans are so much shorter than ours anyway. Do you go for it, don't you? It's a tough call.
Our labs have been 5-6 years apart in age, it just worked out like that. Our third bullmastiff was an only "child" but that's a 140 lb dog we're talking about. Chase was an only child for about 6 years and really seemed happy to have a sibling. We thought Lark was happy enough by herself after Chase died but she's always happy to play with Henry, now that he's past the landshark phase. If Boomer is 12, having an older, calmer dog come into the house is probably a kinder approach than getting a younger, super energetic second pup if you're going to get a second dog. When Chase died 2 summers ago, it was a terrible shock but it was also nice to have another dog upon which to shower our attention, and she was all for that, too.
We haven't hd any terrible medical expenses with any of our dogs, fortunately. Our worst expenses are when we travel and have to board them at a kennel. We had about a 2 year break in dog ownership after Teddy the bullmastiff died and before we got Chase. It made traveling a little easier but aside from that, I missed having a pup around.