Reseller Web Hosting - The small print
You should be aware that a lot of resellers accounts are not the same as if you was to buy just a single account from them. Some companies restrict or allocate the amount of resources you can have so each customer you add the slower your service becomes. This isn't always the case but some do it, furthermore you might find that your reseller account is put on a server with many other resellers meaning you're sharing reseller resources which ultimately will be slower than the other service... Again this is something you have to ask. Some companies offer lower contention on their reseller accounts (i.e. "Low Density" hosting) and this costs much more but can be worth while taking.
You're a Website Designer not a Webmaster/Web Administrator
Another thing to consider is do you want to spend time resolving hosting issues? If you give them accounts they will come to you for support, if I was in your shoes I'd charge monthly managed costs to them if they expect support for their hosting.
Personally I opt to use VPS with webmin and virtualmin. For each 5 customers or so I increase the resources - this is by far the most responsive and best service you can provide without buying a dedicated server. You should of be aware of the upkeep involved and forward these costs monthly to your customers. If a customer isn't willing to pay enough to have you manage just get them to buy a shared account and let them have the responsibility of maintaining it. Remember time is money and try to get some back for hosting/managing.
Managed servers/hosting costs a lot of money from web hosts don't just give them cheap hosting because your making the site for them...