I'd say it is worth less than a +2 to a stat. It counts as a double constitution bonus for hit points, however, it misses out on other things that might benefit from a con increase. You lose out on +1 to con saves or ability checks, barbarians miss out on +1 to AC. I think it's value increases when a PC already has a maxed out constitution as it boosts your hit points beyond what would otherwise be attainable but then if you already have a maxed out constitution then you probably aren't really worried about a low hit point total.
If bought instead of a primary ability increase then you also have to take into account what else you will gain from them. I do feel that 5e is fairly forgiving to players who don't max out their main attack attribute so that not gaining that +1 to hit/damage isn't as big a deal, but there are also all of the extra uses for that attribute that you need to consider (skills, ability checks, saves, initiative if you are dex-focused). I think that basically, unless you are really worried about low hit points or want to create a tanky character that can soak up a lot of damage that the durable feat is a feat that you pick up later in your career, if at all.
I did get mixed up, problem with being away from the books. I actually have no recollection of the durable feat by the sounds of things since that is not ringing any bells at all. Does it let you reroll poor dice rolls?
I'd say that even without the tough feat, your multiclass character would be on par with a cleric as the average of your hit points would be just slightly behind the cleric. However, your reasoning is essentially one of my listed reasons to take the feat, namely, being worried about low hit points. Since you'd be proficient in con saves, the extra +1 to con saves wouldn't be as critical as a straight wizard running into melee so I can understand taking the tough feat.
Durable said:Increase your Constitution score by 1, to a maximum of 20.
When you roll a Hit Die to regain hit points, the minimum number of hit points you regain from the roll equals twice your Constitution modifier (minimum of 2).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.