If your playing at higher levels, or boosting Wis first, I would go magic initiate over polearm. Booming Blade/Green flameblade adds 3d8 minimum, which is more than 1d8+5 especially at high levels, and it doesn’t take your bonus action, which you can use for spiritual weapon.
Booming blade + mobility is also a fun combo.
Polearm wins for under level 11.
Edit: If you dip sorcerer, you can pick up booming blade without a feat, along with the con saves. Put that feat into Warcaster instead of polearm, then run in with spiritual guardian. If they try and get out of the area, they get a nasty OA, if they stay, they get guardianed. Toss spiritual weapon on top when you get enough slots and you'll be doing plenty of damage. Just remember to save a few for healing as needed.