Very good question. You can use both interchangeably but there is a difference in emphasis. With the simple future, we are making a simple prediction or statement about the future, with a time reference e.g. in the next few weeks, tomorrow, next year, by the end of the month. With the future perfect, we are emphasising the result at a particular future time, in this case, "the end of the year". e.g. By the end of the day, I will have taught 8 lessons. I think I will be tired tonight. e.g. I will teach 8 lessons tomorrow and total of 20 by the end of the week.

Just to be clear, when people say "perfect tense", they generally take "perfect" to mean "present perfect" specifically, and they take "tense" to mean a construction combining the auxiliary verb "have" in the present with the main verb as a past participle. And you may be thinking of a sort-of rule out there that says that if you have some time reference in your clause, then you generally shouldn't use that construction, but instead use a simple past form of the main verb alone, so you would say "more joined by the end of 2014" rather than "more have joined by the end of 2014". (That's because present perfect should have some relevance to the present, and if you specify some other time reference, you have broken that connection.) But what you are talking about here -- a construction like "will have left" -- is actually what is sometimes called "future perfect". It's the other way around from the present perfect -- it actually needs a time reference -- such as "by the end of the year" -- and cannot be used without one, even if it is just implied by the context. That's because you are talking about events that will at that time be seen as completed (as marked by "have"), rather than anything that has to do with the present (unless your time reference is "by now" or something equivalent). The events may occur at the time of speaking or they may still be in the future, but they will be in the past and completed by the time point specified.

present no, more likely a future tense