At this moment, there are three "boost stat to 19" items. Two (the headband of intellect boosting Int, and the gauntlets of ogre power boosting Str) are uncommon, and one (the amulet of health boosting Con) is rare.
The thing about "boost to 19" items is they are primarily aimed at the non-specialist. A wizard is unlikely to want a headband of intellect--you're likely shooting for a 20 Intelligence, and you'll hit 18 at level 4, making the item redundant. The gauntlets could be useful for a feat-oriented fighter in the mid-levels, but it still becomes obsolete eventually. To get the biggest boost out of these items, you want to not be invested in that stat.
But the benefits of boosting Int and Str, if they aren't your primary stat, are pretty limited. Int gives you a better Investigation check and a smattering of knowledge skills, and that's about it. Str gives you Athletics, which is nice, and it helps you with grapples and carrying capacity, but it's still pretty small beans. Con, on the other hand, is a very big deal for just about anybody. It's nobody's primary stat, but everybody's secondary. A typical PC can expect to net 2-3 hit points per level out of the amulet. Plus, Con saves are extremely important. Thus, the Str/Int items are uncommon while the Con item is rare.
So, for the other three stats, should the items be uncommon or rare? Well:
The thing about "boost to 19" items is they are primarily aimed at the non-specialist. A wizard is unlikely to want a headband of intellect--you're likely shooting for a 20 Intelligence, and you'll hit 18 at level 4, making the item redundant. The gauntlets could be useful for a feat-oriented fighter in the mid-levels, but it still becomes obsolete eventually. To get the biggest boost out of these items, you want to not be invested in that stat.
But the benefits of boosting Int and Str, if they aren't your primary stat, are pretty limited. Int gives you a better Investigation check and a smattering of knowledge skills, and that's about it. Str gives you Athletics, which is nice, and it helps you with grapples and carrying capacity, but it's still pretty small beans. Con, on the other hand, is a very big deal for just about anybody. It's nobody's primary stat, but everybody's secondary. A typical PC can expect to net 2-3 hit points per level out of the amulet. Plus, Con saves are extremely important. Thus, the Str/Int items are uncommon while the Con item is rare.
So, for the other three stats, should the items be uncommon or rare? Well:
- Dex is notoriously overpowered: It boosts AC, initiative, a major saving throw, lets you escape grapples, and makes you sneaky. A "Dex 19" item should definitely be rare.
- Cha is more on par with Str: It boosts some very useful skills, but it doesn't do a whole lot else. So a "Cha 19" item should be uncommon.
- Wis is a trickier case. Skill-wise, it's about even with Cha in my opinion. Most of the Cha skills are stronger than most of the Wis skills, but Perception blows everything else away--it's the most important skill in the game, and it isn't close. In addition, Wis is a major saving throw while Cha is minor. You could thus make an argument for a "Wis 19" item to be rare... but I'm not quite convinced by that argument. The gap between Wis and Dex/Con is just too wide for me. I'd go with uncommon.
