In terms of effect, all the yeasts do the same thing. They eat the sugar in flour and produce carbon dioxide and alcohol.

With dried and fresh yeast, you get consistency - if all your variables are the same (same type and weight of flour, water, salt and temp) a given weight of yeast will produce a risen dough in the same amount of time.

A starter has its own set of variables that need to be carefully managed if you want to achieve consistent results and so adds either extra complexity (if you’re going to get into managing it properly), or a level of uncertainty to the end result (if you just whack it in and hope for the best).

However, the main thing a starter does that other types of yeast don’t, is add extra complexity of flavour - no matter how well you manage it, it’s going to change the flavour of the end result in a way you wouldn’t get without it.

When you make any bread, it undergoes a period of fermentation, enzymes in the flour are getting to work breaking down proteins and releasing sugars, the yeast is feeding on those sugars and producing CO2 and alcohol. The longer the fermentation goes on, the more sugar will be eaten by the yeast and the more alcohol will be produced. The more sugars left in the dough after fermentation, the more complex, nutty flavours will be present in the final bread.

So fermentation is a bit of a balancing act between leaving the dough long enough for the enzymes to bring out those complex nutty flavours but not leaving it so long that the yeast eats all the sugar and produces so much alcohol that your bread is inedibly sour.

A starter basically allows you to pre-ferment a bit of flour (or a mixture of flours, each bringing their own complex flavours to the party) and, if properly managed, it allows you to add a whole load of additional complex flavours into a dough that, if left to its own devices, would become inedibly sour before it was able to develop similar complexity. The danger is, if you don’t manage your starter correctly, it will get incredibly sour and might end up ruining your final bread.

Personally, I prefer long fermentation using dried yeast over messing about with starters as I like a nuttier tasting bread, rather than sourdough.

TLDR: This has become way longer than I intended but essentially, yeast is yeast - dried/fresh yeast give you consistent results, starters give you a headstart on flavour but with potentially uncertain results.

Well, sourdough is yeast, just not concentrated. But it also has other microbes working, specifically lactobacilli.

It’s the lactobacilli that make sourdough sour.

So, “better” is a matter of your intended results.

Generally, the longer you ferment with sourdough, the more acidic you’ll get. And, generally, feeding the starter more frequently slightly reduces that acidity. It’s about how populous the lactobacilli are when you start your dough. The lactobacilli aren’t quite as fast growing as yeast, so when you feed more often, the yeast is a bit more present, and the lactobacilli a bit less.

Nothing wrong with commercial yeast at all. If that’s what y’all prefer, taste wise, that’s what y’all prefer. It works faster anyway, which is often preferable for a busy home.

It’s really subjective, which one is better. Mostly just improves the flavor and the satisfaction of baking without yeasts. To adjust taste it’s a balance between time and temperature of proofing and quantities and qualities of the levain and dough.

To make the sourdough less sour/acidic you could try a fresher starter, using it as soon as it has fully risen and before it has developed the really tangy smell. Proofing the starter in the fridge may help make this time window easier, and proofing at cold temps should help make it less sour as well. Same with the dough, try shortening the proof time and/or proofing at a lower temperature.

You could also try doing a sourdough with a white bread flour to see if the spelt has a tangier flavor. Or you could try using a preferment with a poolish/biga to see how your wife likes that kind of flavor.