Ok, here's my suggestion. Of course this is only a suggestion, but it worked well for me.

I'd go the custom setting route and store your territory/region data there so that if it needs altered (due to region shifts, additional regions, etc) it'll be easy to change. We set ours up with one entry per state and some identifier of the territory it belongs to. So if we add another territory, we just change the territory numbers on the state entries.You could also just use the user ID of the individual who owns that territory if you'd prefer to go that route.

Once you've got the custom settings set up keyed on the State as the name of each entry, it makes this much easier. Let's assume you set it up with the state name as the name of the records in the list and you've added another field that is the user ID. Then it'd go something like this.

trigger AccountAssignment on Opportunity (after insert,after update){

	//this assumes that your custom setting is "Territory_Assignment_Key__c". It could be anything you choose to name it.
	
	//get the assignment key data from memory. map key is the name field on the custom setting (state code assumed here)
	
	Map<String,Territory_Assignment_Key__c> keyMap = Territory_Assignment_Key__c.getAll(); 
	
	Set<Id> acctIds = new Set<Id>();
	
	for(Opportunity opp : trigger.new){
	
		if(opp.StageName=='Contract' && opp.Number_of_Recruiters__c=='5-9') {
			if(trigger.isInsert(){
				acctIds.add(opp.AccountId);
			}
			//on updates, only need to look at opps where the stage has changed
			else if(opp.StageName != trigger.oldMap.get(opp.Id).StageName){
				acctIds.add(opp.AccountId);
			}
		}
	
	}
	
	//only continue if we need to update some accts
	if(!acctIds.isEmpty()){
	
		List<Account> toUpdate = new List<Account>();
		
		for(Account a : [SELECT Id,OwnerId,BillingState FROM Account WHERE Id IN :acctIds]){
			//no need to change the owner if it's already the right one
			if(keyMap.containsKey(a.BillingState) && a.OwnerId != keyMap.get(a.BillingState)){
				a.OwnerId = keyMap.get(a.BillingState);
				toUpdate.add(a);
			}
		}
		
		if(!toUpdate.isEmpty()){
			Database.SaveResult[] updateResults = Database.update(toUpdate);
		}
	}

}

Hopefully that makes sense. Let me know if it doesn't.