The advantages of a pack and head system are as follows:
Power: You can get power packs that discharge as much as 9600 joules of energy. That is overkill for portraiture and the like, but can come in handy if you're lighting an automobile, industrial equipment, and other large objects.
Lighter heads: Strobe heads are lighter, often much lighter, than monolights. This puts less stress on your stands. Overall, there's less weight in the air, especially if you're booming something. Less weight makes for a safer environment. Depending on the manufacturer, the weight of a complete pack system can be lighter than a comparable number of monolights.
Single trigger source: Everything fires off of the pack. No worrying about slaves. If you want to go wireless you only have to buy a single receiver.
Single control source: All of your power and modelling light controls are right at the pack. No having to climb a ladder or lower a stand to change settings.
Battery power: The battery-powered pack systems are much more integrated and lighter than a comparable monolight system. Many pack systems recycle as quickly on battery as they do when plugged in.
The biggest advantage that I think monolights have over pack and head systems is redundancy. If you have three monolights and one dies, you still have two with which to shoot. If your pack dies, you're done until you get it repaired. You can, of course, buy a spare power pack to keep around as a spare, but that drives up the cost. However, 2 2400 joule power packs can be nice to throw at certain problems.
You can find monlight kits for less money than a pack and head kit. But I think this is due to the fact that you just don't find really cheap pack and head manufacturers. Most (but not all) packs are engineered to fire all day every day, week in and week out. When you find monolights that are built to be used the same way the price difference evaporates.
I have, at one time or another, used just about every major manufacturer's lights. And I will tell you that with lighting you get what you pay for. There are definate reasons that a 300J monolight from manufacturer A costs $300 and a 300J monolight from manufacturer B costs $700. The reasons may not be important to you, but in general the more expensive lights are much more consistent in power output at low power levels and vary much less in color temperature across their power range.
To the original poster: when you say that you want something that is mobile, what are you specifically looking to do?
People who do stupid things with dangerous substances often die! -me