If your math skills are very good and you want to work as hard as you would as a CEO in an early stage startup, your pretty much guaranteed to make a LOT more money in finance. Ditto if your solely a quant and not a trader or a quant/trader.
That being said, if your startup makes a glorified text messaging program that Facebook buys for a cool billion $$$, you'll be sitting pretty. Statistically, you're much more likely to succeed and make a lot of money in finance. Each company is different, but many firms let traders keep 3-6% of their PnL as end of year bonuses. Extremely good compensation is the name of the game in finance, but in a startup it is like playing roulette. Your wins can be a lot bigger, but so can your losses.
If it was me, I'd go finance, but I'm a technologist at heart and absolutely love the tech I'm able to work on with the tech budget of a solid financial firm.