Sometimes it's much easier to have different web sites for different markets because it will keep everything seperated. For example, if you offer "budget hosting" and "premium hosting" many people that you target for premium hosting would see your budget plans and think they were paying to much. (Just try explaining to someone why they should pay you $40 per month for premium hosting with premium support when you offer the same plan with little support for $5 per month ! 9 times out of 10 they'll just assume they will get the same support and go for the cheap plan)
Anyway, the downside is that unless you link between the sites, you won't have any overflow traffic between the sites, and you will need to market two different brands.
Overall, I would say it really depends on how different the two target markets are, and how they are different. With the example above, having one target "premium" and one "budget" , I'd completely seperate them. However if you were say targeting premium hosting and reseller hosting, I'd say keep them together, as you are not likely to "lose" any customers to the other market.
Another tricky thing to do is to just do a one-way link between the sites. So, for the example above, make a link from budget hosting to premium hosting, but not the other way (so customers who come to the premium hosting won't be tempted to go for the budget plans, but the people at the budget site can go for the premium plans).
Anyway, I'm kind of rambling here, so I think I'll just stop.
I would use just one site..
Develop good content for each individual page and packages you offer.
Or use sub domains.
Administering multiple web-sites and marketing each and developing seperate content etc would take alot more time IMO.
I also wouldn't know how you could have your cc processor or ssl certificate on multiple sites? You might find you would have to buy different ssl certificates etc if you are e-commerce ?