Any suggestions/advices on how to decide one vs. another?
http://www.woopra.com is a great tool but their pricing is a bit high for the amount of events you described in your question. They also include analytics for web.
http://www.kilometer.io looks very promising as it targets to analytics for startups and aims to be very simple. Event based analytics such as this are really the best ones.
What are the current problems you are having with MixPanel?
disclosure: I work there, but there's no reason you shouldn't try us out if you're looking for an alternative.
Our free plan is for 1 million monthly events: https://amplitude.com/pricing
all the basics you're used to, like Funnels and Retention, plus some other cool stuff like unlimited user timelines (this is extra as part of Mixpanel's People plan)and Event Flows.