Brace yourself for everyone to tell you it's a bad idea for one of a million reasons. But if you want to spend your money on it, by all means give it a go.
Both firms are established and what they're selling you is their routes – tried and tested – plus their services to sort out the logistics of booking it all for you, so you don't have to bother. What you get is directions and notes on what to look for along the way. What you don't get is company (where tour companies usually score, as having other riders to talk with in the bar of an evening is what's really different to going on your own) or flexibility - which you wouldn't have if you were prebooking everything anyway.
It is an expensive way of doing things - you'll note that half the content on the travel section is either pre-planned trips or people recommending ways to refine trips other people are planning. But if you don't want to plan your own trip, it's a way of getting it done. There are guide books, magazines and websites stuffed with routes that can be glued together to provide complete preplanned trips. Indeed several (ahem, i
ncluding my own) offer complete tours that are effectively self-guided packages, minus the fact that you have to find/book the hotels yourself.
The thing to establish is whether the tour these firms offer are off-the-shelf self-guided packages (ie one-size fits all, days may be too long or short for you, will probably err on the side of safe-and-easy roads which might not suit if you want narrow-and-gnarly) or whether they'll custom-plan a route to suit how many hours a day you want, on the kind of roads you want. That's a service I'd say really is worth paying for, as long as they know their onions and can deliver a bespoke package that ticks your boxes and takes you to places you wouldn't find for yourself. However, the chances are that the self-guided trip is the one they'd normally lead a group on, in marked-up-map-plus-GPX-file-and-notes form.
Either way, you're looking at established firms so you should have some confidence: they wouldn't have lasted if they didn't deliver good riding for people. I've dealt with MCi and they're good. I haven't dealt with Magellan directly but I know people who've done and enjoyed group trips with them.
Only you can decide whether paying for their services is the right thing for you to do.