11:16 a.m. on May 30, 2006 (EDT)
I have had an XGK for many years and never had a serious problem (had to replace the G jet after about 10 years of hard use). And I have had a Primus MFS (the predecessor of the Omni) for about 12 years. When using liquid fuels, I find them about equal. I have used white gas, autogas, kerosene, autodiesel, jet fuel in the XGK (change the jets, of course), and in the MFS white gas and kerosene. The one advantage of the MFS (and Omni) is that you can also use butane mix canisters (the ones with the industry standard threaded coupling). The one problem I had with the MFS was that the hose developed a leak right at the swage, probably due to stuffing the stove into the pack too hard a few too many times. The Omni has the hose attachment a little changed with the simmer valve added, so probably wouldn't have that problem. Besides, with a bit more attention to how I pack the stove, I haven't had the problem since.
The problem people have with the MSR pumps is rough handling. In over 30 years of having MSR stoves, I have never had a problem with one of their pumps, although I have seen other people break them, usually by doing something brilliant like stepping on the pump, and once by a careless arrangement of the windscreen that directed heat from the stove onto the pump - melted it, producing a spectacular jet of flame. Take care of your stove, and it will serve you well.
I will note that if you go to places that are frequented by serious expeditions, MSR stoves (mostly XGKs) and Primus far outnumber Optimus. I will also note that every stove failure I have witnessed has been the result of carelessness, other poor handling, or lack of maintenance of the stoves. Even the problems that are common to MSR's Dragonfly can be prevented by proper maintenance (an MSR tech rep once told me that if people would read and follow the directions that come with their stoves, he would be out of a job). Overpriming is probably the biggest source of poor performance (I see this all the time in the winter camping and backpacking courses I teach). Lack of cleaning is probably second, especially when using 3rd world fuels, which are often contaminated (by the way, do NOT use marine diesel in your stove unless you are very very sure of its cleanliness). Just plain carelessness with handling and spills is another major source of problems.
Be sure you take an expedition-level maintenance kit with you, with spare jets for both white gas and kerosene. You can get the kerosene almost anywhere, but you may not be able to get the white gas everywhere. Although the XGK and Omni can burn autogas, you probably should not use them except in a dire emergency. These are leaded in most parts of the world, which introduces both the lead poisoning problem and clogs the stove quickly, meaning frequent cleanings. Carry several spare filters (the ones that go on the fuel line). Use separate fuel bottles for kerosene and white gas.
And learn how to do the regular maintenance, including cleaning the fuel lines (the "pipe cleaner" trick) and jets (remove the jet, so the junk doesn't just fall back into the fuel line). And, of course, learn to properly prime the stove (if your priming flame is more than 2-3 inches high, you are overpriming).
I am not as familiar with Optimus stoves, but, as I noted above, they certainly are not as popular among climbers in the major mountains. I haven't been tempted to get an Optimus, even though I have been offered several freebies.