Watercolours are generally available in tube and pan forms.
I use watercolour pans primarily but after using the Daniel Smith watercolour tubes, I'm beginning to prefer the latter.
Below are the main differences between the watercolour tube vs pan that I've experienced from using them so far.
There's no noticeable difference between vibrancy of hues from tubes or pans. The main determining factor would come from the grade of your paint, whether it's artist or student grade, and the second factor comes from your paper which may dull down the colour.
I also see no difference when tube paints are dried, then re-wet for use again.
Main advantage for tube paints is they are much easier to mix for larger washes. Pans in their cake form are harder and do not dissolve as easily as tube paints. For this reason, when painting large pieces, working from pans can be very inconvenient, be it full pan or half pan.
Tube paints generally re-wet easily, but it depends on the particular colour of course (e.g. Daniel Smith Viridian dries to a hard rock). Subsequent drying and re-wetting do not affect the brilliance of the colour as well.
Because tube paints dissolve more readily, it's also easier and faster to mix them to higher concentration.
For pans, even when sprayed with water, it requires more moistening to soften the paint before it can be used. For selected colours or formulation, they can be even more difficult to dissolve than other colours in the pan set (e.g. Schmincke's Pthalo Blue pan).
Watercolour pans are much easier to bring around, as most come sold in box sets. Tubes may be sold in box sets as well, but those sets consist typically smaller tubes, and it's usually more economical to get larger 15ml tubes.
For outdoor usage, care have to be taken to dry the pans before packing as they may leak when packed in the bag that tends to get knocked around. More so if you're using tube paints in pans.
Tubes are way more economical than pans. You can refill half pans several times with a single 15ml tube.
Damage to brush
This applies to pans. Since the paint is dried hard, it's more likely to damage the brush through prolong usage of pulling the pigment out. I've see brush hair get stuck in the pans because of that.
Having said that, my sable brush is still well and alive after two years, but it does look quite worn already.
Schmincke uses the same formulation for their tubes and pans. Schmincke pans are said to be the made from their tube paints, which are left to dry before another layer is added. From a visual inspection, their pans do look more glossy than other manufacturers. But in actual use, they have the same characteristics as pans, i.e. they are as easy to dissolve as tube paint.
After I finish using my current batch of pans, I'll be switching to filling empty pans with tube paints. They are just easier to wet and make huge washes with. And it's also more economical.
For those looking to building their own set of colours, I recommend getting empty watercolour boxes and pans, then choose your own watercolour in tube form to fill them.