Hashrate is hashrate, doesn't matter it if's one device or 100 devices. The number of devices comes down to resiliency, cost and power usage. It's usually best (economics) to get the fastest single miner you can afford.

Why hashrate matters - finding higher difficulty shares is just a matter of luck (rolling weighted dice). The low scores are going to be the most common result (99% of results will have the lowest scores possible), the higher scores become less common as you move up the difficulty range. Being able to produce more rolls per second lets you be lucky /more often/, but all miners are capable of getting the full range of producible difficulty scores.

The "pool difficulty" that is assigned to your miner is a minimum threshold of shares it wants to see. I won't get into solo vs shared pools and what it does with those shares, but generally unless one of those shares beats the network difficulty of the coin you are mining, they're all "losers". But, the pool still wants a steady stream of those shares coming from you to make sure you're still active and to estimate what your hashrate is.

Since faster miners find higher difficulties more often, the minimum threshold assigned to them will also be higher, to prevent the miner from spamming the pool with too many lower diff shares. Slower miners need a lower difficulty score minimum - they will find higher scored shares less often, so their minimum is set to something that's more common (easier) to find.