They are both very similar types of services. You enter a tweet, then you state the date and time you want it to actually be posted on Twitter. When the time comes – that tweet gets posted to your Twitter feed.
I actually like both of these a bit more than Timely.is – which I had previously been using – as these services allow me to set the exact time. Timely fed out my tweets throughout the day based on how many tweets/day I wanted (I think I could go in and manually edit the time, but that was an extra step).
I found Twuffer to be more intuitive of these two services – mainly because setting the time and date was point and click – rather than having to click on the date / time box and physically back space or delete the current date and time and enter a new one.
One tested strategy is that grouping tweets in threes is more effective than single tweets – and with Twuffer -this could easily be accomplished. I could put three tweets all at the exact same time, or spread them out 5 minutes apart. I think that is something I will test – to see the most effective timing method.
I look forward to testing engagement of my followers at different times of day and days of week as I actively tweet more often.
Will keep you updated on any results I get.