Hell, I'd be number one fan of these two: Menya Mappen & Oiden.

In my previous short trip to Sydney two weeks ago, these two were the most satisying cheap-eats that I came here so often, 4 times in 5 days, lol. So there goes reason number one: it's super delicious and affordable, especially if you're on a budget trip, OR... if you came to Sydney to watch Coldplay 2 days in a row and the next thing you realize....you're running out of cash :|

So, which one's which?

Both restaurants are similar in terms of concept: Japanese self-serve tempura chain. Difference is, Mappen highlights noodle (udon & soba) whereas Oiden highlights their rice bowl dishes more. And it's a battle of Deep Fry everywhere, in which we have Mappen's light-coated crispy tempura batter vs Oiden's thick panko crumbs. Your choice!

MAPPEN:

Ontama Mentaiko Bukkake (Udon) $5.90
Hot noodle with Tsukedashi sauce with chili cod roe & half-boiled egg
Ontama Bukkake (Soba) $4.90
Hot noodle with Tsukedashi sauce & half-boiled egg
Ontama Bukkake (Udon) $4.90 + lots of chopped shallots, tempura flakes, and lots lots of nanami togarashi (Japanese assorted chili pepper) :p
Self-serve Tempura Bar - heaven!
Vegetable kakiage tempura $2.80
Takoyaki $2
Chikuwa (fish cake) tempura $1.90 and sweet potato tempura $1

Mappen's awesome full menu:

Mappen's awesome place and Japanese vintage decor:

OIDEN:

Ontama Beef $5.90 - stewed beef & half-boiled egg on rice
Ontama Beef $5.90 with Chikuwa Fry Tempura $1.90
Ontama Takana Salmon $3.90 (medium bowl)
Salmon flake, mustard pickles, dried seaweed, and half boiled egg on rice. + lots of tempura flakes & chopped shallots!
Ontama Curry $4.90 - curry & half-boiled egg on rice
Ontama Curry with additional Shrimp Stick in breadcrumbs $2.90
Self-serve Tempura Bar - heaven!
Free flow tempura flakes and chopped shallots, at your convenience :)

Oiden's awesome full menu (they put it up at front of the shop, btw):

What it loooks like inside Oiden:

VERDICT:
This is not a battle. I LOVE THEM BOTH, but technically I'd always prefer udon than rice, and tempura batter rather than panko crumbs. So, Mappen slightly wins!

If you happen to visit Sydney someday, give it a try. It's my type of comfort food.

Mappen Sydney (CBD)
Shop 11, 537-551
George St, Sydney
NSW 2000
http://www.mappen.com.au/shop/sydney-cbd/index.html

Oiden
Beside Mappen. No, seriously, they stand side by side!
http://oiden.com.au/