They can both do a lot of similar things, but if I were speaking broadly, I would say that I think of PNW more like a clock generator and Little Nerd more like a clock manipulator. PNW can also manipulate incoming clocks, and Little Nerd can be your clock source, but you use up a channel of Little Nerd if you have it generate your clock.
If you’re using Marbles you have a clock source, though you may want to clock Marbles externally rather than using it as your primary source. Rene doesn’t provide its own clock unless you are repeatedly tapping touch plates, which is an interesting definition of clock 
The answer is always, it depends what you’re trying to do and how you want to do it. But very generally, PNW is a more capable module and is a better value both for money and size, since it can provide a lot more functionality than Little Nerd. I wouldn’t go with Little Nerd unless a) you wanted to build a kit - that’s why I have mine - and b) you wanted the specific interface it provides. I wouldn’t view Little Nerd as a financial savings unless you have your clock generation 100% sorted elsewhere and you’re viewing this module strictly as clocking ‘spice’.
On the other hand, in a small system I think Little Nerd would be a more interesting companion to Marbles, if Marbles was your main driver. In a small system I would view Marbles, or PNW, as the “driver”. They’re not exactly redundant, and you can clock either Marbles from PNW or vice versa, but, both modules kind of want to “be in charge”, and in particular I would feel if Marbles is your main driver, PNW would be somewhat under-utilized as a clock divider/multiplier.
This is my view, and other folks have others which may work for them!
If nothing else, if you get PNW and don’t like it, you will stand a much better chance of reselling it than you would a DIY Little Nerd.
(I keep adding things as I think of them, sorry for the edits!)
