The MGS will be using the rifled barrel, the exact same ones we have on the Leo C2 presently. In fact, the barrels we have in stock will be the ones mounted, or kept in reserve. A mix of L7A1/L7A3 and M68.
When firing ammunition with velocities in excess of 1,000 m/s, rounds are fin stabilized regardless of the barrel they are fired from. The 105 mm rifled barrel fires fin stabilized APFSDS and HEAT rounds. To keep the rifling from inducing spin to the round, which would degrade the fin performance somewhat, a "slip ring" is incorporated to the round. The ring spins, the round doesn't (much).
The difference in accuracy with APFSDS is negligible, modern FCS and barrels are extremely accurate. It doesn't really matter whether the barrel is smoothbore or rifled.
Slower rounds, such as HE type rounds, are spin stabilized. This is because the round flies much slower, and the fins would have to be too large to stabilize the round in flight. The 120/125 mm smoothbores do not fire HE type ammunitions because of this. They are limited to APFSDS and HEAT ammunition. The rifled barrels have the advantage of being able to fire smoke and HE type ammunition.
It boils down to what you want from the tank. The MGS, from what I gather, will be used a lot in the Infantry support role, so we would want the capability of firing support ammunitions. The Germans and Americans see tanks primarily for killing other armoured vehicles. That's one of the reasons the British have been so reluctant to get rid of their 120mm rifled cannon, they want to be able to fire HE and smoke.
The BMP3 uses a rifled barrel, despite the Soviets not much liking rifled barrels. That's because the HE-Frag round is very slow, and therefore requires spin stabilization.
Hope this helps.