Kilimanjaro is a nice walk that takes around a week +/- two days and takes place at a moderate altitude. Summit day is a pleasant climb that leaves you feeling refreshed.
Elbrus is comparable to Kili, except it may have harsher weather and greater snowfall. However, if the weather is fine, the trek will be a little more strenuous (due to the snow).
Aconcagua, on the other side, is about 7000 meters tall, significantly higher than Kilimanjaro and Elbrus. Windy weather is also a possibility. The biggest issue will be the altitude, which will make the summit day much more difficult than a nice trip on Kili or a quick snow climb on Elbrus.
All common routes are non-technical.