Here I will put some facts of each aircraft so you can form yourself your own opinion:
-The Su-25 has a much greater speed than the A-10 due to the use of turbojets instead of turbofans (975 vs 682 km/h)
-The A-10s GE engine burns a lot less fuel thanks its high-bypass turbofan design while offering the same thrust as the outdated turbojets on the SU-25.
-Both can also use Diesel fuel.
-The Turbofan engines on the A-10 present a cooler exhaust then the straight jets on the SU-25. This will make it harder for an IR missile to track onto it. As well mounting the engines where they are on the A-10 also shield them from observation from the ground.
-The smaller size of Su-25 and its greater speed gives it a higher chance to remain undetected by enemy (but in the case it is detected, the A-10 has better chance to survive because of its well separated engines and its IR shielding & modern countermeasures)
-Su-25 has a maximum load capacity of 4400 kg vs 7200 kg of the thunderbolt
-The Su-25 has a lower loiter capacity & combat radius while carrying the maximum combat load: 1250 km vs 2000 km for the A-10
-The A-10's anti-tank 30mm cannon with its depleted uranium bullets causes more destruction then the one on the SU-25.
-The A-10 can take extreme damage and still remain aloft thanks to its heavy armor, widely spaced, high mounted engines and unique tail configuration(half the tail can get blown off). The pilot is better protected thanks to heavy armor around the cockpit and bulletproof canopy. The A-10 also has self sealing fuel tanks. It can land safely without its landing gear down.
And all this is for the basic A-10. A-10C (With PE upgrade) puts it beyond the CAS capability of any aircraft ever built.
Last... just look at the tank chewer rounds!
Sukhoi-25 is the best for CAS among turbojets, but turboprops (many to choose from), as discussed by
@Kunal Biswas, have some big advantages over Sukhoi-25.
