I prefer AGI to SCI.
First of all, I like to work on these adventure games as more of nostalgic reasons. Let's face it, AGI and SCI are not cutting edge in graphics and sound and, this being the only criteria, people would not choose them over the newer games. However, people are playing them and I think this is due in part to the fact that they remind people of that first computer they got -- people like to reminisce and this archaic-style of big pixels, limited colors, and internal speaker-quality blips & beeps brings people back to those early innocent years. AGI marks the beginning of this era and many of us enjoyed watching Sierra get better at producing games in this same format -- from KQ1 & SQ1 to KQ4(yes, there is an AGI version),SQ2, and Goldrush. Although Sierra went on to produce great games using SCI, in my mind nothing compares to those originals.
Secondly, I've already spent some time learning how to make games in AGI and have even started some. I don't want to have to abandon the work already done and re-learn another tool. Some say it's slightly tougher. Maybe it is, maybe it isn't. Some people have a naturally easier time working with code while other struggle all the way. Personally, I haven't looked at SCI enough to make any definite opinion, however, I'm sure there are differences and I really don't want to confuse myself until I finish some of these AGI games. I have a hard enough time with code as it is.
Lastly, one of the most time consuming parts of developing an AGI game is the artwork. I know of several projects that went undone simply because drawing realistic background scenes became too burdensome. As tough as it is, AGI is quite forgiving on the detail so that many who do not consider themselves artists can produce impressive scenes. The reason is that a few pixels in AGI can represent an entire control panel and anyone playing the game will believe it. However, in SCI, there is a greater detail level which requires more experienced skill in drawing in order to produce the background scenes. While it is true that SCI has the ability to display even more impressive scenes because of this greater detail level, it also adds a greater burden to an already burdensome task.
Anyway, that's my 2 cents.