It really depends on what you're selling. Paypal doesn't have a strong fraud system in place so they will shut you down at the drop of a hat if you're not selling $15 beaded necklaces... Paypal's basic options is essentially one where company with it's own merchant account allows other smaller merchants to process under their own account. So they make the rules, offer less control and offer less protection. Merchant accounts are setup by merchant account providers that can tap you into your own merchant account and process directly with a Visa/MC approved bank - you can only do this with a real merchant account. Merchant providers can also recommend Payment gateways to you that will allow you to use your merchant account online.
Starting out if you're working with very small merchants then Paypal is probably a good option unless you're selling "high risk" goods/services. But once you even suspect that your client's businesss is growing you definitely need to look into setting up merchant accounts for them.