I have experience of fitting both
Quality wise I have had many more issues with Wren than Howdens. You also have the added headache (from a fitters perspective) that once you have identified an issue you have to wait for a replacement, with Howdens you just take it to your local depot and sort out the return on the way in the following morning. On one or two occasions the defective part from Wren has been replaced with another defective part multiple times which was a major arse ache for me and my clients.
Overall I would go with Howdens. But if there is a style you like from wren it will still be good there just may be some more bumps along the way.