There are currently 5 different Redis add-ons available on the Heroku platform.
Redis To Go, Redis Cloud, openredis, MyRedis and RedisGreen
Which do you choose? I imagine there's a fairly long tail of Heroku apps that use one of the free Redis tiers offered by the providers that have one, but there must also be (me included) a pool of developers who want to develop and test on a single Heroku instance using a free tier, and then ramp everything up to a billable level once they go live.
For me the decision was based on who gives you the most for free.
Whilst I have no experience in running a hosted Redis service and what impact free tiers have on profitability, it seems unlikely that when faced with 5 options a developer is going to go for one they can't try for free. That would rule out openredis and RedisGreen straight away (chepeast tier $10 and $169 per month, respectively).
With the remaining 3 options you will be left to compare and contrast simultaneous connections and database size. MyRedis gives you 5Mb and 3 connections, which knock them out of the race.
So then you're left with RedisCloud and Redis To Go. Both have the same number of simulatenous connections (10) which leaves RedisCloud as our winner because they give you a 20Mb instance over 10Mb.
Does this make logical sense? Perhaps not, but I can't be the only one who has gone through this thought process. It might make more sense to look at the higher tiers which will match your intended capacity, but there's a great opportunity to get customers in by offering a good quality free tier.
When I went live and ramped up, did I research the next tier up? Nope, just hit upgrade on my RedisCloud free tier.