This doesn't seem to be going anywhere since I was last here so I may as well drop my bias.
EMIYA
- Killed Herakles 6 times apparently without UBW
- Blocked death flight Gae Bolg with Rho Aias
- Walked off getting impaled by several swords to the back for some time, despite being running on fumes without a master
- Can supposedly trace Excalibur, but it's a stupid move since it'll kill himself as well
- Can fire powerful enough shots that can supposedly make Saber's knees give out
Mirou
- NLBW'd his way through a group of heroic spirits, when even one of those can pressure Illya, and a group can hold down her entire team
- Blocked a hail of arrows from several mud spirits with Rho Aias
- Traced conceptual weapons though only its form, not its power
- While crippled, bleeding out and just got out from imprisonment, shot an arrow strong enough to pierce through Mjolnir's lightning
- Dodged Excalibur point-blank
- Tanked Enuma Elish's physical attack after his Miyu roids was cut
Key points
- EMIYA has never fought Arturia at 100% for some reason (he was caught off-guard by her appearance, he was more intent on shooting Hrunting at Shirou, etc.)
- Prillya's card servants are weaker than their original servant counterparts, mud servants are even weaker
- Despite point #2, the mud servants were still giving tremendous trouble to Illya's team
All things considered, I'm giving this one to Mirou because despite his lacking experience relative to EMIYA, he thinks better under stress, which is a major factor in battle. EMIYA has thinking outside battle down pat though since he did just play chess against everyone and they didn't even know.
Also, his feats like using Overedged to stab an existing KnB through the enemy shield, switching weapons to get in his opponent's guard (e.g. using a **** huge sword to push away Girlgamesh's shields), his tracing conceptual weapons despite being near-crippled, reading into Kuro's movements despite only just meeting her to set up a killshot against Girlgamesh, etc. are what I think is better suited to a versus match, and thus, I side with Mirou.
On another note, did we get confirmation that EMIYA is the one he summoned to the card? Or was it Mumei?