Frederick or +Hp/-Spd Spring Alfonse?

As you're running this green horse on a cavalier team, I'm going to go under the assumption that you are running Hone/Fortify and will be applying those buffs in my analysis.

HP: So miniscule it doesn't even matter.
Atk: Same
Spd: Buffed Alfons hits 36 which is already enough to do the job a green cav has to do, but ultimately doesn't make that much of a difference as blue mages can still wreck him and the only threatening lance heroes are all equal or higher speed. These points in BST are better spent in a different area, such as...
Def: Whereas Alfone spends some points in Spd, Freddy spends them in a stat he cares much more about in Defense. Both candidates were probably not going to see many follow-ups on their end anyway and Def is a good skill to bolster. You can make up for the lack of his speed with a Breaker skill to cover a weak area or QR.
Res: If you play the game correctly then these units really shouldn't have to worry about tanking mages that often. Alfonse has 4 res that he really doesn't need. But this generally doesn't matter.
General: Your Alfonse is locked to certain IVs while Frederick who can appear as a 3* can be rerolled several times to obtain optimal ones. He also has great merge potential which can fix his weak areas and ultimately bring him much further above S!Alfonse than he already is (atleast in my opinion). This also helps if you plan on getting into/staying in Tier 20 with Horses.

TL;DR I believe Frederick is better. If you love Alfonse you can just build him and he can do his job just fine, but if you invest in Frederick he will grow over time and surpass Alfonse. If you want you can give Freddy the spoon as a useful weapon, or give Def Smoke to someone else who could probably use it better.