From what I saw I'd advise betking, it's a great way to invest.
Not too complicated, not a high demand of minimumm price.
Just put what you can in it and wait for the profit
Betking is solid, but the problem is they already have huge bankroll, so if you're looking to invest small amount, you'll get tiny share of profits. That means nothing on it's own though, sometimes it's better to have smaller share in higher profits.
That's why you should look at wagered/bankroll ratio.
For Betking, weekly wagered to bankroll is only 24%, while for SafeDice it's almost 870% (higher % is more attractive to investor). The difference is huge.
Data from https://dicesites.com/Of course there are other factors to consider: what's the house edge, max allowed kelly, house commission on every bet etc.