Also how should I know witch miner should I buy? More efficient one? more hashpower? or what?
The AMHash miners have lower fees = more profit. Not a bad choice.
But they usually have a worse efficiency than Gen 1 miners (it's not much, 99,xx%).
Furthermore AMHash is
real mining hardware provided by ASIC manufacturers who confirmed that Hashie rented hashrate from them, for Gen 1 there is no proof for real mining activity (as far as I know).
Looks like more you invest better % you get.
Stated in the miners market:
Volume Discounts
20,000 GH+: 1% bonus hashpower
50,000 GH+: 3% bonus hashpower
100,000 GH+: 5% bonus hashpower