It's worth noting that a new edition of 40k is dropping on the 17th, and the game is going to have many HUGE changes. Also, you might find you get a better response on sites like reddit (I'd recommend /r/warhammer40k/ ) or dedicated 40k forums like Dakkadakka where you can get tailored opinions.
Regardless the best army to start with is one that appeals to you (fluff, playstyle, paintscheme, models depending on YOUR preference) ... If you like the armies in the starter box a great (economic) way to start is to split one w/ a friend but if you don't like the starter armies there are plenty of other options.
Ultimately how you get started is an intensely personal decision.