For $'s sake and convenience I would recommend getting another set of pedals and just stick with SPD's.
If you plan on doing more than 20 - 30 clicks in a session then road shoes benefits start to kick in. I find road shoes hold your feet better allowing you to put more power through the cranks on the uphills to maintain speed and gear without windmilling like you do on a MTB.
I have road pedals/shoes for my roadie because the shop did an awesome deal on a very light pair of road shoes that are super comfy with some 105 pedals.
As an aside - I ride out with my Epic jersey and water bottles on the road for training sessions and get ignored by the other roadies
If you plan on doing more than 20 - 30 clicks in a session then road shoes benefits start to kick in. I find road shoes hold your feet better allowing you to put more power through the cranks on the uphills to maintain speed and gear without windmilling like you do on a MTB.
I have road pedals/shoes for my roadie because the shop did an awesome deal on a very light pair of road shoes that are super comfy with some 105 pedals.
As an aside - I ride out with my Epic jersey and water bottles on the road for training sessions and get ignored by the other roadies