I currently use zerodha for investing in mutual funds. Both offer direct funds. There are no transaction charges with zerodha coin, no demat charges either, but they do charge a flat monthly fee of Rs.50, regardless of the number of transactions or funds you invest in. With piggy, as per their website, the fee is Rs.30 per transaction for every time you buy a fund, and Rs.100 per year for every fund you SIP. RS.3 for…
You are forgetting that Zerodha needs you have a demat account and then you pay for it. Because you can't do MF with Zerodha w/o a demat account (can do so with many other brokers). Also, for this monthly charge Zerodha doesn't provide any analytics or advisory while many other direct fund providers, at a similar monthly cost, do (and couple of them won't even ask you to have a demat a/c and pay for it).
But there are no transaction charges in zerodha on buying equity (delivery) or MF. So it is still cheaper than piggy for someone wishing to make a large number of transactions or someone who doesn't want an SIP but wishes to invest manually every month.
Also, if you need a demat account for some other purpose - like investing in ETFs or equities then maybe it makes more sense for some people.
If all you are doing is an SIP with a couple of mutual funds, which is what most people around me do, piggy is probably cheaper. You can always have a zerodha (or equivalent) account for equity/ETFs/etc. and piggy for MFs.
But for someone like me with slightly different requirements, zerodha would be cheaper. I have a variable income so SIPs are not an option for me. I invest money as a percentage of my income so the amounts vary every month. In this case zerodha turns out to be cheaper despite the demat charges.