Both peaks are in Nepal and although the country is home to 8 of the world ten highest peaks, it is also home to many smaller peaks between 5800m and 7000m. Mera Peak, along with some other peaks in Nepal such as Island Peak (Imja Tse), is defined by Nepal Mountaineering Association (NMA) as a Trekking Peak. There are currently 15 of these Group “B” Climbing Peaks, defined as such because it only takes one day of actual climbing, that they require no technical expertise to climb and are under 7000m.
Unfortunately, this has led many to believe that there is no difficulty involved. This is not the case. Anything involving the use of ropes, crampons, climbing gear, climbing skill, snow gullies etc can cause difficulty, even for just for one day.
Both mountains are classified an Alpine PD grade, so you need to have proper technical skills to ascend the peaks such as knowledge in cramponing, harnessing and ice-axing. Many people tackle these peaks without any prior expertise, and receive on the site training the day before the ascent.
This is what makes the two peaks similar but now for what makes them different.
Mera Peak is almost 300m higher than Island peak, at 6,476m/ 21,246 feet above sea level vs Island Peak at 6,189m/ 20,305 feet.
Mera is also a lot more remote and takes you through some of the most spectacular scenery with tarn lakes, massive valleys and high passes. It also takes longer to get to and overall the day to day trekking is much steeper.
Island Peak essentially follows the main Everest base camp trail, diverting up the Chukkung Valley from Dingboche and therefore a more popular route on the early days.
As the route to Mera is more remote, there are less tea houses and in places, no tea houses, so you will spend your nights in tents. On the Island Peak trek you can stay in tea houses all the way to Chukkung before you start camping.
So essentially, Mera is tougher to get to and higher and requires tents, but climbing to the actual summit is easier going than Island Peak. The top section of Island Peak or Imja Tse, is slightly more technical than Mera with the last 400-500 m section requiring climbing up a steep ice wall and then up several snow rises. The lower part is also very rocky. On Mera, it is easier going but the final 600-700m requires the technical work.
And what about the summit? Well the views from the summit of Mera are unsurpassed as you look about several peaks over 8000m, however they are all far away. On Island Peak, you are quite up-close and personal to the peaks, giving you more of a sense of being among them.
At the end of the day it depends on what you want out, how high you want to go and how long you have.