For spellcasters, the 1 point increase to your save DC is notably more valuable and powerful than the +1 to your attack bonus; when debating whether to raise your core ability score s a caster, think about your save DC, rather than your hit bonus, since save DC is much harder to raise, in general. Things like having advantage from various sources that can grant it, things like bless - there are numerous things that will give you an edge in hitting with attack rolls, but most of them do not also benefit your save DC, since that's not you rolling, but the target rolling to save.
As a simple maths consideration - sources of advantage mean you get two chances at hitting your target, which means that your actual odds of hitting with at least one of those rolls is substantially higher. by contrast, far fewer sources can give enemies disadvantage on their saving throws against your spells, in general, and after levle 5, you'll begin coming across more than a few things that have natural advantages with saves against your magic. As you gain levels as well, you'll find that most caster spells work through saving throws, rather than attack rolls - attack roll spells are in the extreme minority, in fact - though a larger proportion of cantrips have them - and so you'll be using your save Dc a lot as the game progresses, potentially far more than your spell attack bonus.
The increase from 15 to 16 for a spell save is worth more, as well, not just because of how much easier it is to boost the odds with attack rolls, but also because of the much reduced bracket for enemy saves, compared to the range of AC they might have; you'll be fighting enemies with AC ratings anywhere from 10 to 20, in the first ~8 levels of play, and a +1 measured against that scale is a smaller ratio than a +1 measured against a scale between 0 to 5, which will be the rough range of save bonuses that your targets might have in that same level bracket. So, the real question, for a caster, is more akin to "is raising my spell save DC to 16 at level 4 more valuable than [Appealing Feat]?"
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When deciding if a feat is more valuable than the ability score increase... that's a much fuzzier question, and mostly comes down to how you want to play, and what benefits the feat will give you, and what value you personally put on those effects and benefits. I wish I could be more help with this part, but feats really are a personal thing of playstyle and preference; you'll never go wrong with capping out your core ability score first, definitely, but if you play a certain way, and find a particular feat appealing to you as a result, and think you'll be able to get and use its benefits regularly with your play style, then take it.
