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 every time you buy a liquid fund.

So piggy turns out to be cheaper for most people who would start an SIP with 2-3 mutual funds. Since I guess most people won't have needs that extend beyond this, piggy is probably the cheapest platform to buy mutual funds. However, for someone expecting a larger number of transactions or SIP with more than 6 funds, zerodha would turn out to be cheaper. Also, if instead of an SIP, for some reason you wish to invest a different amount every month, zerodha would turn out to be cheaper.

A big advantage of having a zerodha account is that I can also invest in index ETFs which I feel are much safer than mutual funds for the long term. Index mutual funds have very high expense ratios of more than 1%, but in comparison the index ETFs have lower expenses, like the NIFTYBEES etf has an expense ratio of just 0.1%. The SBI NIFTY etf has an expense ratio of 0.05%. There are no exit loads or transaction charges. These are passive funds that just track the basket of companies in the NIFTY index.

I think index ETFs should also form an important portion of a person's long term/retirement investment portfolio, apart from mutual funds.