Horford is a better bet than Howard at this stage of their careers
As Howard has relied much on his athleticism so he's game is going down
While horford is based on skill so will age much gradually
Disagree since Howard has length and physical strength that tend to allow you to remain competitive even as you get older - and he is an elite rebounder, a skill that tends to stick around as guys age.
Horford has modest size, and smaller bigs tend to decline early because they rely so much on skills (which fade as you get older).
For example KG as he got older has remained a very good defensive player and an excellent rebounder, but his offensive game has dropped off a cliff.
If you look at very long big men who were great defensive/rebounding players (KG, Rasheed, Duncan, Mutombo, Camby, etc) you'll find those guys tend to remain pretty effective to around the age of 34-35. They are able to use their size and length to continue to make an impact on the boards and on defence long after their athleticism and offensive skills fade.
However if you look at modestly sized, not especially athletic, offensive minded bigs (e.g. Boozer, West, Randolph) you'll tend to find few players of that description who managed to maintain key roles at the ages of 33-35. You have Karl Malone and Charles Barkley, but it gets tough beyond that. Once these guys lose mobility and the body breaks down, it gets harder to maintain an edge offensively against younger, larger and more mobile guys - they often have to fall back to being just spot up jump shooters, and even those jumpers often fade in effectiveness with time.
Assuming injuries don't cut his career shot, I suspect Howard's game holds up better to time than Horford's.