In terms of comparing the two parks, here's what the 2016 lineup will comprise of (based on news & rumours):
Thorpe park
Leading attractions 5 coasters & a dark ride (average throughout 1100)
Supporting attractions 2 smaller coasters, 10 flats, a drop tower & 4 water rides
Other attractions 1 walk through & a4D cinema
Relaxing/non ride areas Sunken Garden & that hill near the dome
Breakdown ride level 4- severe
Alton Towers
Leading attractions 6 coasters (one of which is getting a heavy re-theme) & CBeebies Land
Supporting attractions RMT, Spinball, Congo, Battle Galleons, Duel
Other attractions 5 flat rides, Sky ride, Sharkbair Reef, 2 shows & a 4D cinema
Relaxing/non ride areas Towers Ruins, The Gardens
Breakdown level 3- adequate
Whilst Thorpe have more filler attractions, Towers still offer more all round additions, not to mention higher throughputs too, where towers supporting rides likely average 700 pph whilst Thorpe's only do 500 PPH. In addition to this, there's more places to go if Towers is busy compared to Thorpe where if there's 18k guests at Thorpe, the park will be packed, but Towers will be moderately busy in comparison.
However for Thorpe's tiny size which is around 25-30 acres to Towers's 500+ acres it does quite well in all fairness.