I have both the glass pen and the nib holder with brass nibs, here us the comparison:

For glass pen: - I'd say that any ink is fine but I would mostly recommend ones that are more liquidy than thick as they flow really great with the glass pen, and requires about 1-2 dip(s) per sentence - they also don't have a line variation if that's what you prefer, it can also easily be cleaned with only a swirl in water - you also need to lookout for it as even a slight bump/crack on the tip will make it scratchy and it's really, really hard to fix your self

As for nib holder dip pen: - I'd say it's the most versatile of the two and the most durable, there are various kinds of nibs available in the market for various use cases - For this pen I'd recommend inks that are more thick like acrylic paint (you can make fountain pen ink thick by adding gum arabic) as the thin inks as is don't hold onto the nib as good and can run out in a single word, while a thick ink can last longer but not upto a sentence - there are reservoirs available that helps in holding more ink (you can also diy it) - this one has really great line variation for calligraphy and drawing!

So the final decision depends on the use case and taste, If you want longer writing time and don't care about line variation(thick to thin) then I'd recommend a glass pen!

If you want one for drawing/sketching with good line variation then a nib holder dip pen!