Dmitri, I have both an XGK (early 80's model) and an Optimus Nova (Brunton distributed them in the US for a while).

These are two very different stoves. The XGK is a simple design that does one thing well-burn virtually any kind of liquid fuel at full blast. It is easy to fix and rarely breaks down. It was designed as a mountaineering stove, not a gourmet cooking stove.

The Nova is more versatile-it will simmer, but it is finicky and prone to clog up. When it works, it works okay, but mine has been trouble. Other people love them. When I take it, I carry a small canister stove (Primus Micron) as a backup.

Primus has a stove that will burn liquid fuel as well as canisters. I have only seen one once at REI.

The Vapor is very expensive. I have no idea why.

There are a lot of threads here on stoves. Use the search engine and look back a year or more.