It's level dependant, since HAM doesn't change. You should compare the number of HP you get from Toughness against the assumed number of times you'll get hit per day that HAM activates
Lv 1 +2 HP <1 HAM Hit
Lv 2 +4 HP <2 HAM Hits
Lv 3 +6 HP =2 HAM Hits
Lv 4 +8 HP <3 HAM Hits
Lv 5 +10 HP <4 HAM Hits
Lv 6 +12 HP =4 HAM Hits
Lv 7 +14 HP <5 HAM Hits
Lv 8 +16 HP <6 HAM Hits
Lv 9 +18 HP =6 HAM Hits
Lv 10 +20 HP <7 HAM Hit
Lv 11 +22 HP <8 HAM Hits
Lv 12 +24 HP =8 HAM Hits
Lv 13 +26 HP <9 HAM Hits
Lv 14 +28 HP <10 HAM Hits
Lv 15 +30 HP =10 HAM Hits
Lv 16 +32 HP <11 HAM Hits
Lv 17 +34 HP <12 HAM Hits
Lv 18 +36 HP =12 HAM Hits
Lv 19 +38 HP <13 HAM Hits
Lv 20 +40 HP < 14 HAM Hits
As we can see, HAM is best at lower levels, and then tapers off in the mid to higher levels. Generally speaking, I'd recommend HAM up to level 8. After level 8, you're cost:benefit ratio becomes lower, and Toughness becomes more viable. If the +1 ASI from HAM would increase your Con to an even modifier (and your DM allows retroactive HP), then I would say that up to level 10 would be an acceptable gain for HAM, but that's the furthest I'd recommend.
Honestly, if HP is your goal, then I'd start with HAM, increase Con to max, then take toughness at level 12+. You'll be an undying machine