<apex:page standardController="Opportunity" extensions="dispatcherOpportunityEditController"
    action="{!nullValue(redir.url, urlFor($Action.Opportunity.Edit, opportunity.id, null, true))}">
</apex:page>

public class dispatcherOpportunityEditController
{
    public dispatcherOpportunityEditController(ApexPages.StandardController controller)
    {
        this.controller = controller;
    }

    public PageReference getRedir()
    {
        List<Opportunity> oppList = new OpportunitiesSelector().selectById(new Set<Id>{ ApexPages.currentPage().getParameters().get('id') });

        PageReference newPage;

       if (oppList[0].recordtypeid == '012800000007gBQ')
        {
            newPage = Page.SubscriptionAmendmentEdit;
        }
        else
        {
            newPage = new PageReference('/' + oppList[0].id + '/e');
            newPage.getParameters().put('nooverride', '1');
        }

        newPage.getParameters().put('id', oppList[0].id);
        newPage.getParameters().put('retURL', '/' + oppList[0].Id);
        return newPage.setRedirect(true);
    }

    private final ApexPages.StandardController controller;
}