Learn Russian: Какая оценка важнее?

The basic adjective is важный (important). To make the comparative "more important", we drop the basic ending -ый and add the comparative suffix -ее. This form is super convenient because it doesn't change for gender or number!

Is it possible to say важней instead of важнее?

Yes! Words that form their comparative with -ее can often take the ending -ей instead. They mean exactly the same thing. Важнее is more standard for writing, while важней is very common in everyday speech.

Since оценка is a feminine noun, why doesn't the comparative adjective have a feminine ending?

Because Russian comparatives that end in -ее or -ей are invariable. This means they never change to match the noun's gender, number, or case. Whether the noun is masculine, feminine, or plural, the comparative stays exactly the same: важнее.

Does какая mean "what" or "which" here?

It means "which" in this sentence. The word какой (and its feminine form какая) can translate to "what", "what kind of", or "which". Because the sentence is comparing things to find out which one has more value, "which" makes the most sense.