solid lifters are for the higher performamce builds ... they have the advantage of not using engine oil to keep them working properly and can rev a lot higher with the problems you get with lifters 'pumping up' Hydraulic lifters use engine oil to keep themselves expanded and making sure that the correct clearances ( ie: none ) are kept. Solid lifters are basically that, solid and the adjustment is in the valve train and a manual job.
The are higher maintenence but if you want a serious build I personally think you can't go past them ... unless you get into roller but that's another story alltogether.
They also have the advantage of being rattly and a lot of people think your motor is worn out and clagged ..