Try to think about it, investing in arena will give you a permanent seats, when you play long enough you'll probably have alot of extra money unless you wanted to be competitive all the way, therefore you'll built the arena seats eventually, thus, if you have extra money, why not built it now to earn more possible income if your arena seats is limiting your ticket revenue.
Investing in players involve risk, and requires rare chances, one of the chance is you'll need one owner who will have his players to start the auction with a really low price, and then you'll have to consider properly to determine on whether will there be people who is willing to buy the player with higher price(10% more than the price you used to buy the player to offset with the tax you have to pay for selling immediately, the tax will slowly be reduce over time and stop at 3%, 97% selling price), and then you'll need another chance at that time which happen to have totally no other people is there to bid war with you.