I think I should clear few points to avoid any misunderstanding. First of all cleanliness of air is nothing to do with the type of dryer and with proper filters you can get the cleanest air possible. Moreover, traces of oil is detrimental to desiccant dryer where as refrigerated air dryer can tackle this quite easily. You must have a filter in the outlet of the desiccant dryer where as refrigerated air dryer may not require this.
Refrigerated air dryers can dry air to almost -250C ADP (PDP of 30C at 10 kg/cm2)but desiccant dryers can dry air better than refrigerated air dryers if the pressure of air is common.
Desiccant air dryer initial cost will be low but operational cost may be high because of regeneration. The size of the dryer, dewpoint, budget and availability of service are the factor you have to consider when selecting a dryer.
Regards,