Hi CZ -
I guess it all depends on what you especially want to see.
Cards on the table, I have a huge soft spot for Wasdale in the Lakes, and especially the Wasdale Inn and the attached Ritson’s Bar. But, and it’s a big but, there’s nothing else there except some of the Lake District’s finest peaks and scenery and it’s deepest, loneliest lake (no boat trips). So unless you were desperate to go hiking, then there’s not much else to detain you. It is also difficult to get to - the best way is via the west coast road (A595) and in via Santon Bridge. The journey from there to Keswick would also be long and complex, no real straight through way and the shorter routes will involve some hair raising driving on narrow roads (masterful understatement!).
There is more of interest in North Wales with the coastal castles and it has the mountains in the Snowdonia peaks if scenery is what you are after. So while I would choose Wasdale over Wales as a personal choice, as a first time visitor to these shores, North Wales might well be your best choice. Also in my view the Lake District rewards detailed exploration, especially if you are into hiking, and perhaps a return to the Lakes later for a less hurried trip would be the thing!
Neither the Lakes or Snowdonia are at their brilliant best if the weather is unkind, but that is the one thing you cannot pre-book or plan ahead for. Fingers crossed on that front!
Have a great trip whatever you decide!
Ian