Борщ со сметаной вкуснее, чем борщ без сметаны.

Со is a pronunciation-friendly variant of с.

Russian often uses со instead of с before awkward consonant combinations. Since сметаной begins with см-, со сметаной sounds smoother than с сметаной.

The meaning is the same: with sour cream.

Because the prepositions require different cases.

• с / со meaning with takes the instrumental case
  
  • сметана → сметаной
• без meaning without takes the genitive case
  
  • сметана → сметаны

So:

• со сметаной = with sour cream
• без сметаны = without sour cream

It is the instrumental singular of сметана.

This is the case commonly used after с / со when it means with in the sense of accompaniment.

A very common pattern is:

• чай с лимоном = tea with lemon
• кофе с молоком = coffee with milk
• борщ со сметаной = borscht with sour cream

It is the genitive singular of сметана.

The preposition без always takes the genitive, so:

• без сахара = without sugar
• без молока = without milk
• без сметаны = without sour cream

Вкуснее is the comparative form of вкусный, which means tasty.

So:

• вкусный = tasty
• вкуснее = tastier

Russian often uses this one-word comparative where English uses more or the -er form.

No. Simple comparative forms like вкуснее do not agree with the noun.

So вкуснее stays the same with:

• masculine nouns
• feminine nouns
• neuter nouns
• plural nouns

That is different from a normal adjective like вкусный / вкусная / вкусное / вкусные, which does change.

Чем means than in comparisons.

The pattern is:

• X вкуснее, чем Y = X is tastier than Y

So in this sentence, чем introduces the second thing being compared:

• борщ со сметаной compared with
• борщ без сметаны

Not always, but repeating it makes the sentence very clear and balanced.

You could also hear:

• Борщ со сметаной вкуснее, чем без сметаны.
• Борщ со сметаной вкуснее, чем без неё.

These are also natural. But repeating борщ helps avoid ambiguity and sounds nicely complete.

Because Russian normally omits the verb to be in the present tense.

So Russian says something that looks more like:

• Borscht with sour cream tastier than borscht without sour cream

But it means:

• Borscht with sour cream is tastier than borscht without sour cream

This is completely normal in Russian.

Yes. Russian can also make comparisons by using the noun after the comparative in the genitive:

• Борщ со сметаной вкуснее борща без сметаны.

That is grammatical. However, the version with чем is often clearer and very common in everyday speech.

Yes. Russian word order is flexible, and changing it changes emphasis more than basic meaning.

The given sentence is a neutral, straightforward version:

• Борщ со сметаной вкуснее, чем борщ без сметаны.

Other possible versions include:

• Со сметаной борщ вкуснее, чем без сметаны.
  This emphasizes with sour cream.

• Борщ вкуснее со сметаной, чем без сметаны.
  This emphasizes the comparison between the two ways of eating it.

Here борщ stays in its basic dictionary form, the nominative singular.

That is because each half of the comparison refers to borscht as the thing being talked about:

• борщ со сметаной
• борщ без сметаны

With чем, Russian often keeps the compared noun in the form that fits its own part of the sentence, instead of forcing it into one special comparison form.

A good learner-friendly stress pattern is:

Борщ со сметАной вкуснЕе, чем борщ без сметАны.

A few pronunciation notes:

• борщ is one syllable
• вкуснее is stressed on -не-
• сметана is stressed on -та-, so:
  • сметАной
  • сметАны

Also, щ in борщ is a soft sh-like sound. An approximate English-friendly pronunciation of борщ is borshch.

Because Russian has no articles.

So борщ can mean:

• borscht
• the borscht
• a bowl of borscht

The exact interpretation comes from context. In this sentence, it sounds like a general statement about borscht as a dish.