Chalk me up as another person who thinks spells ought to be rated lower than feats.
First, casters have a pretty easy time adjusting their selection of spells as they go. Beyond the basic ability to change loadout every single day, they (or at least some of them) can recover spent slots, prepare spells in a matter of minutes (remember that you don't have to prepare all spells at once), use ritual versions, or employ scrolls and potions. A feat, meanwhile, is selected once at the expense of increasing ability scores and is set in stone forever after.
Not all casters though. A bard doesn't has quite that ability (and I expect sorcerer won't either), bards also pick a spell and they become set in stone forever (or at least as long as it takes to level up and retrain, just like feats)
Second, feats are rare. You don't really get that many of them. The Fighter is arguably the feat poster child and he gets seven of them at most by the time he's level 18. The Mage has for more tools at his disposal than that.
Equally at 18th level a Bard only has 10 spells known while a Mage knows at least 38
Third, feats gobble up your ability score improvements. You are making a very real opportunity cost by selecting a feat. Spells don't have that. They only compete with other spells, but it's not as if simply holding prepared spells reduces your stats (now there's an idea for a game...).
Granted, not as high an oportunity cost for bards, but still an important opportunity cost, if a bard has picked lots of spells that end up never being relevant he/she has lost a lot of effective power, for an spontaneous caster not knowing the right spell is as good as not having any spell slot, you still end up not casting anything
Fourth, there is an argument that spell selection has to be carefully made because some spells can wind up being useless. That's a double-sided sword though that punishes feats far more severely, because feats also have to be selected beforehand... and can still wind up being of little help. But due to the factors listed above, spells can both adjust more quickly (or even at all) and there's a good chance that where one spell is useless, another one prevails. Did the Mage prepare Invisibility only to be foiled by a portcullis? Should he have prepared Spider Climb instead? Gosh, if only if you could prepare and use both at once... Oh wait, that's right, you can. At level three.
For a bard every spell known counts, and she cannot change them on the run at all. Did your bard learned Charm person, sleep, disguise self and comprehend languages just to be thrown against hordes and hordes of undead?, though luck, hope you make a better selection for the next campaign
A feat that never came up during an adventure was an unfortunate waste. A spell that never came up during an adventure was a trivially small investment, and was replaced the next morning with a spell that did come in handy.
For a bard a spell that never came up during an adventure is equally a waste, or even worse, because unlike a feat, spellcasting is a class ability, if a class x with Loremaster, never had a chance to use the extra languages or lores, well he/she can still fully function otherwise, if a bard's spell selection results useless for a given adventure, she functions at half capacity for the full adventure.
