Which upgrade path for banana farm is better? Left or Right?

I know that the left hand side upgrade for banana farm is the research facility which produces a lot of bananas while the investment bank gives interest. So, which one is more cost-efficient given very little space in the map? I'm going for approximately 100 rounds or so.

It depends on the situation.

The 2|4 farm is cheaper, and can be bought and paid back sooner. In fact, it's $7500 cheaper than a 4|2. It produces $1180 each round with a 20% interest rate after the round, which is a little bit more than half what the 4|2 produces, but, after 7 rounds after acquisition (assuming you haven't withdrawn) it will have payed itself back and produce $2300. The 2|4 will pay for itself sooner, however, the funds are not as sufficient as the 4|2.

The 4|2 farm is a lot more expensive, and cannot be paid back as fast, but if it is purchased early on, it can have a positive effect on gameplay. After the same 7 rounds, it will be $2400 short of paying itself back, but it produces a constant $3000 each round wich quickly adds up and will be very beneficial for late-round gameplay.

In short, The 2|4 farm is for short term investments, and the 4|2 farm is for long term investments. The 2|4 will produce some quickly, the 4|2 will produce more after a longer period of time. I personally prefer the 2|4, but this site is for answers, not discussions.

This is the math that you are looking for.

Essentially the answer to your question is, it depends. If you are looking for a quick return on your money soon, go plantation. If you are looking for the most money at some point , go with bank.

Space on the map doesn't matter, they take up the same space regardless of upgrades.