I run task teams managing rights of way and my prefference is for the sthil FS400 - none of the lighter models in the sthil range will be up to driving a brushcutting blade through small trees etc (note that you need the 3 pointed brush knife blade not the 2 blade or plastic/nylon heads) while the heavier models are unreasonably heavy for the extra power (though if you want to use a clearing saw you might want the FS450)
I dont know anything about huskie brushcutters but they are an equally good make - dont be tempted to buy cheaper brands because they will break more often.
I'd also note that for five acres you definitely need the full body harness not the shoulder sling.
In fact if you arent used to it brushcutting five acres will really take it out of you - and you might be better off looking at hiring a power scythe or two wheel tractor and flail to do the initial cut (you can then stay on top of it with a brushcutter - buying a decent power scythe is likely to be prohibitively expensive) - Something like this
BCS 32" Flail mower