Managed server (from my experience with the 20 I have) come at different levels of 'managed' but regardless, as a new host or small host, you will usually get better and more personallized support with a reseller plan from your favorite host. It takes the 'server' level out of your worries and allows you to focus more on selling, supporting your client base and learning the biz. I would never recommend starting with a server if you haven't at least hosted some sites for a while as a reseller or something. It is like riding a bike right away without training wheels. Give you and your comany a chance to grow into the 10 speed.
Not on a colocated and on a leased option some do provide a managed solution but you are looking at around $350 to $500 a month for that. Most cover hardware but when it comes to software you are on your own.
These offers you see here on the forums for dedicated servers do not include a managed option. Do you know how to install apache? or set up a virtual host file in httpd.conf? If not expect to pay about $90.00 to $160.00 an hour for someone to do it. Owning a server can get quite expensive if you don't know what you are doing. Not to mention what your customers are going to do while your server is down and you are trying to learn.
I do not know what your case is so it is hard to answer, if you know how to manage a server and you think you can get the client base to pay for it a dedicated is better in the sense that you are in control and do not have to depend on someone else if it goes down.
Primarily if you no clients to start off with getting a dedicated server will only serve to get you stressed out to getting clients on the box to offset the monthly cost, starting out with a reseller plan, buys you some time to do this with the lower cost to start one, until you have enough clients to make it feasible to get a dedicated server