As far as I am aware, unit trusts are dual priced. OEICs have single pricing, that is midway between the two. That's just how they are structured. If there is no initial charge, you will pay no initial charge.
As far as selection goes, it is probably much of muchness these days. Skandia always used to be the market leader as far as range went, although I'd imagine now most supermarkets are pretty much the same.
One thing to look out for, as a result of the RDR you are likely to see supermarkets offering a wider range, including ETFs.