Comparing Red Mahjong with Hong Kong, MCO, Zung Jung, etc.   

Red Mahjong (2-4 players)

Red Mahjong resembles gin rummy as well as a popular mahjong-like card game MHING except that Red Mahjong uses traditional tiles and a much simpler scoring system. 

Despite its simplicity, Red Mahjong  is a rich game of strategy that you can learn and start playing in 5 minutes!

There are no differences in rules between 4 and 2 player games except for picking up discards from more players. The rules become simpler with just 2 players - no more worrying about whether you're collecting for a pung or a chow.

The main difference with 2 players is in the game's strategy:  with 2 players, you can really go for the big scoring hands in a way you can't with more players.

With just one opponent it can be quite a tight game: you have to watch your discards carefully. With more than 2 players, you can be less watchful with discards, but with just 2 players it really helps to watch what your opponent might be collecting.

In later stages of the game, you won't have a choice of what to discard, but when you do, it's really worth thinking it through.

If you're going out, you really don't want it to be for less than 32 points minimum. You have, however, try to go for something achievable - if you go for too good a hand you'll never complete it before your opponent finishes their hand.  The key is to judge how close your opponent is and whether or not to go Mahjong with a hand that is not quite perfect.

Flowers and Seasons change the nature of the hand:  if one player has a pile of these tiles, then the best strategy for both players is to just go out as quickly as possible.

At times, going Mahjong as soon as possible can be a good play if you think your partner has a powerful hand. You may be able to tell that from observing the tiles on the table in front of your opponent.

With more than 2 players, going Mahjong tends to happen way too often, so that's where setting Minimums (e.g. 5 point minimums) can be helpful.

Red mahjong is an excellent game when played with just 2 players, with quite a lot to think about, and more strategy than their might appear at first glance. Red Mahjong becomes a tactical rummy-like card game with 2 players. 

Comparing Red Mahjong with Hong Kong Mahjong (HKOS)

Red Mahjong can be easily played with 2-4 players. Red Mahjong has a very simple but unique hand scoring system which is a bit similar to a MHING game. 

Comparing Hong Kong Mahjong (HKOS) with Chinese Classical Mahjong (MCO)
In most Chinatowns of big cities, you can buy mahjong sets that come with little booklets of Chinese Classical mahjong scoring rules translated into very poetic English.

It can become a daunting task, however, for a mahjong novice new to mahjong terminology to learn scoring rules from these booklets. With the Chinese Classical scoring, there are many situations that will keep doubling the hand value to up to 8-times its scoring value.
With doubling, your hand value may grow to 1000 points or more (most hands have maximumums of 120, 250, 500, etc.) Compared to Chinese Classical scoring, Hong Kong system is easier because the points start with small numbers and go up to their maximum values.

Comparing Hong Kong (HKOS) scoring with Zung Jung scoring
The Zung Jung scoring system increases the available number of hand patterns from 12 to 44. So with Zung Jung system, there are more patterns to remember. Some players argue, however, that Zung Jung system offers more opportunities compared with the Hong Kong system.
In Zung Jung mahjong, there is no emphasis on self-draw hands and there is a strong discouragement for going out with "chicken hands". As there are a greater number of patterns to match against (especially in the "Terminal", "Identical" and "Similar" categories), with only a minimal increase in effort spent building the hand, there are more chances to score a hand higher than a "chicken-hand".
The HKOS scoring system, however, is an easier system to learn compared to Chinese Classical mahjong or Zung Jung mahjong. Mah Jong is primarily a game of patterns and all these special hands are only extra patterns to learn. In HKOS system there are only 12 main patterns that need to be memorized. Knowledge of the special hands' patterns make mahjong a more strategic game.
Despite a smaller number of special hands' patterns, Hong Kong scoring system offers a wealth of opportunities in combination with a relatively simple scoring system. In addition, any penalties, minimums and maximums may be optional in Hong Kong mahjong.