Numbers
Alethkar has access to over a hundred thousand soldiers on its own. Jah Keved is supposed to be about equal to it, and they probably make up about half of the Rosharian military power. So 450,000 would be a rough estimate of Roshar's strength as of WoK.
In contrast, the Final Empire has access to around a quarter million koloss plus their human armies, which probably are about equal to Roshar's based on how a quarter of the Final empire could muster 120,000 soldiers total (Cett+Yomen's armies).
Scadrian Advantages
However, quantity will not be the only things deciding the war. In terms of soldier quality Scadrial is obviously superior (koloss are said to equal 3 human soldiers). Additionally, there are roughly two dozen mistborn and hundreds of each type of misting as of the first Mistborn book. These would probably make a huge difference on the battlefield as soothers and rioters could rout the Roshar army almost singlehandedly. Coinshots and Thugs would make a huge difference as well;as we have seen, a few coinshots can tear through dozens of soldiers. Also, the Coinshots would be useful in bringing down shardbearers. The Lord Ruler would also make a huge difference, as he can soothe or riot thousands of people at once. However, he can be killed by a shardblade, so it would be unwise for him to fight on the front lines of a Scadrian army (same goes for inquisitors)
Rosharian Advantages
Roshar has two main advantages: Shardbearers and Soulcasters. Soulcasters would be extremely useful in supplying an army and negating the need for a supply line like the one that Scadrial would need. A drawn-out war would therefore favor Roshar and its near-infinite food supply. However, Roshar's main advantage in a pitched battle is its shardbearers. Roshar currently has access to around a hundred shardblades and plates. A shardbearer can mow through hundreds of enemy soldiers in minutes. However, shardbearers are not invincible, as seen in the tower battle at the end of WoK. In normal combat, Shardbearers would have a massive demoralizing effect on an enemy army. However, emotional allomancy will negate the most harmful effects of this, and a duralumin steelpush anchored by iron compounding(inquisitors and TLR are capable of this) will easily tear through shardplate.
Conclusion
In conclusion, I think Scadrial would win due to superior numbers and more common magic, as well as their ability to avoid demoralization and counter shardbearers.
Sorry for the long answer, I couldn't help myself