And I agree with Prism and Mistwell: The comparison here is between the Toughness feat and +2 Con. Bringing Dex into it is just complicating the issue for no reason.
There is a very good reason.
The comparison is really between the +4 Con and the Tough feat. The first costs two ASI slots. The second costs one ASI slot, but does not give the +2 to Con saves/checks. The second option loses the +2 to Con saves/checks in order to get either another feat, or another +2 to another ability score.
+2 to another ability score is heads and shoulders above a +2 to Cons saves and checks.
That is the reason I brought Dex into the picture.
Tough < +4 Con - +2 Con saves/checks for one less ASI slot.
Tough + +2 Dex > +4 Con - +2 Con saves/checks for the same ASI slot cost. This is not only better, it's a lot better for PCs that get targeted in combat a lot (maybe their DM targets casters), especially ones in light or no armor.
Alternatively, Tough + Lightly Armor > +4 Con - +2 Con saves/checks for the same ASI slot cost. Again, much better for a PC in no armor that might get attacked frequently. For the same slot cost, the one PC gets the same hit points and +2 for AC and +1 to Dex. The other PC gets the same hit points and +2 Con saves/checks.
The same could be said for a different ability score, but Dex is the big elephant in the room because it helps AC, Init, and Stealth.
There are four levels of defense in order for nasty Con attacks and/or for concentration.
1) Get total cover. If the PC cannot be attacked, then he will not get hit.
2) Improve AC with ASIs and/or with cover and/or with spells. If the PC is hit less, then he will need to make fewer Con saves.
3) Get Warcaster to protect against concentration saves. For casters who use a lot of concentration spells, these will fail more than Con saves due to nasty monster attacks since the situation will occur more frequently
4) Improve Con
Improving Con is the least of these because it's the last line of defense, and hence, helps infrequently. Bumping up the other defenses helps more.
Comparing +4 Con to Tough plus another ASI is a much closer and accurate comparison. The PCs have the same number of hit points in those cases, so going down because of hit points alone is not a factor.
Comparing +2 Con to Tough is a logical fallacy. It tries to keep the same number of ASIs whereas the important thing is to keep the same number of hit points in order to reduce other variables.
For a player who wants 2 more hit points per level (the purpose of Tough), Tough plus another ASI (feat or ability scores) is much better than +4 Con. Considerably better.
If a player does not need or want 2 more hit points per level, then +2 Con is the obvious choice and Tough is out of the question. Comparing these two is idiotic. It's only when the player wants 2 more hit points per level that a comparison makes sense.