The batteries you choose will have pros and cons, and in many cases, the deciding factor is cost (don’t make that mistake). For me, as well as @Namasaki the decision to go LiPo is discharge capability. As you may already know @Lospalaz the C-rating of a lipo is the manufacturer’s claimed max. sustained-current output without causing damage to a cell’s chemistry. Depending on how you ride, you may very easily exceed 78 Amps at times, but probably not often. If you are planning on smaller capacity packs, GO HIGH C-RATING, or wish you had later.
A capacitor pack will not mitigate voltage-sag, at least not for any sustained period. They are more for smoothing out the current spikes associated with switched DC motors (brushless motors). The Cap-pack you linked above is the one I bought, and it’s awesome. It will increase performance, but not necessarily in a way that you will feel. What it WILL do is increase the life-span of your ESC, clarify your RF signal, look awesome, and give you more instant-power!
So my opinion: Capacitors? - get some! 5200mAh 15c LiPos? you should reconsider, but they may be suitable for you…