The 155 howitzer because I've never seen one in 1/48th. Is there a kit to start from or is it all scratch? After all, you say you have an M5 HST, so if you finish the 155mm Gun then you have to find or build a Mack NO or an M4 HST. Both would seem rather daunting compared to completing a howitzer.
Actually, build the one that you feel most attracted to, you will do better work that way and (hopefully) have a bit of fun.