I'm looking for a Burgundy wine to serve with a meal: is this Ratafia what I'm looking for?

Appearance: Bright amber color with golden highlights.
Nose: Intense aromas of candied fruits, honey, dried apricot, and ripe citrus.
Palate: Soft and sweet attack, balanced between fruity richness and fresh acidity, with a long and slightly spicy finish.

This product is classified as a spirit, not as a Burgundy wine. So if you're mainly looking for a bottle of wine to serve with a meal, don't rely on the name "Burgundy" alone.

Which should I choose: this Ratafia or the Very Old Marc de Bourgogne "Ultreîa"?

These two products have different designations: Ratafia for one, Marc de Bourgogne for the other. They should not be considered interchangeable; however, the information provided is not enough to tell which one better suits your tastes.

Does the "non-vintage" designation mean that this Ratafia is young?

No: the absence of a vintage does not indicate its age. On its own, it therefore does not tell you whether to open it soon or keep it.

Is the 50 cl bottle a good choice if I want to limit the amount I open?

This size lets you choose a volume of 50 cl. That is a practical consideration if you want to limit the amount you open, but it does not tell you how long the bottle will keep after opening.

If I want to discover Champy Boris, should I start with this Ratafia or its Marc?

There is no way to recommend an order based on the information available alone: neither the aromatic profile nor the strength of these products is specified. You can find them in the Champy Boris selection, without assuming that one is more approachable than the other.