Regarding capacity, Poma buttons tend to run faster than T-Bars, which at least partly compensates for doubling up on T-Bars. Does anybody know how the typical line spacing compares? That would also have an impact.
Line speed doesn't determine capacity, it's hanger spacing in seconds. In most cases the T-bars have more capacity, a lot of the Poma's here are pretty poor capacity wise because the have relatively short racks or are underpowered. On CairnGorm really only the Daylodge Poma which has a large rack, powerful drive and thus (normally) a short light cycle can really rival the t-bars for capacity.
For a given physical carrier spacing distance a faster line speed will increase the nominal capacity because it will reduce the interval in seconds. In theory a t-bar can shift 1200ppl per hour, the highest capacity Pomas (like the Daylodge) have a nominal capacity of 1000ppl per hour.
On CairnGorm uplift capacity has been further trashed over the years in addition to by reducing the number of lifts, because slower linespeeds with same carrier spacing (Cas T-bar notable in last 3 years) and much greater carrier spacing at similar line speed (Ciste and Ptarmigan T-bar, when the spring boxes were replaced with oilboxes in 2001). The same happened back in 1998 to the Ciste Chair when capacity was reduced from 1000pp per hour to 600ppl per hour by having far fewer new carriers than old ones.
Moving ahead a few years, once the Meall Odhar chair is installed, season's like the one just gone will be significantly better than 2015 was at Glenshee because the problem of snow nearly all coming from the West will be mitigated by much easier circulation on Meall Odhar and thus also to/from Coire Fionn and much greater uplift capacity on the Cairnwell which tends to like Coire Fionn get the best of the snow in very Westerly seasons.
Which means all things being equal unless there is sudden shift on CairnGorm, given reasonable snow conditions at both, Glenshee is going to wipe the floor with CairnGorm.