Saving Throws, Again

I still fret over saving throws. I'm dead certain that non-critical issues, and critical ones for NPCs, should be handled by a d6 roll rather than a d20, because it allows me to fold these saves into damage rolls or other effects rolls. I'm also pretty much sold on the ideas Delta's D&D Hot Spot presented about granularity, so I've retreated from an all-d6 approach. Save vs. poison, petrification, or dragon's breath should probably be on a d20, to allow narrow escapes.

No, my inner debate is about saving throw categories versus ability checks. The debate runs something like this:

• Saving throw categories are roll-over, just like attacks;
• ... But ability checks are roll-under, making survival easier in many situations, especially at 1st level.
  
• Saving throw categories allow for variation by danger type -- magic can be more dangerous than poison;
• ... But ability checks allow variation among characters: some will be better at dodging dragon breath or worse at resisting poison than others, regardless of class and level.
• ... But saving throw categories allow for variation by class, which means that fighters will find some things riskier than other classes would;
• ... But ability checks could have the same variation, with class-specific bonuses.
• ... But saving throw categories also allow for variation by level and improvement over time, with some classes improving much faster than others;
• ... But ability checks could again do the same thing, just by adopting the bonuses in Target 20.
• ... But if you're going to adopt the Target 20 type, class, and level bonuses, why not just use Target 20, keeping saving throws recognizable to a wide variety of people?
• ... But then ability variation is lost; anyways, some situations seem like they shouldn't be easier to resist with level.

What I think I'll wind up doing is using ability checks for the d6-based saves (non-critical and NPC saves,) but Target 20 for critical PC saves other than physical injuries, disease, and other non-supernatural, physical causes. The latter are d20 roll-under attribute checks. The full level bonus will only apply to the magical/unnatural situations; for the d6-based and roll-under d20-based saves, the level bonus is limited to comparing attacker and defender levels/HD and giving +1 to the die roll if the attacker's is higher, +1 to the target number if the defender's is higher. If the difference is extreme, it's a 2-point penalty.

I'll keep writing saves as "Con save" or "Dex save", though, because I will also allow a +1 bonus if the defender has a high attribute (4 or higher.) For unnatural attacks, the player's description of how the character avoids the attack can result in a further bonus. If a player throws a bucket of water right as a dragon breaths fire, it won't stop the attack, but it might be worth a +1 for a high Int.