As with most topics regarding locomotive design and locomotive marketing, it's not that simple. I'll try to be brief.
The market for such locomotives was initially plumbed by ALCO in 1963 with its C-420. This was a 2000 HP turbocharged locomotive with all the most modern features. It began to sell rather decently.
Electro-Motive responded in its new "1966 line" with the GP-38. This had a normally aspirated engine, and direct-current transmission. While the major focus of the line was the new, high-horsepower turbocharged and alternator-rectifier locomotives the GP-38 did begin to sell too.
General Electric finally responded to this market in 1968 with the beginning of production of its U23 line. These units were turbocharged, and rated 2250 HP for traction. It seems that GE decided to up the ante in terms of horsepower in order to make itself a niche in this market.
At this point, it's obvious that in one sense ALCO and GE had an advantage over the EMD unit in terms of ability to produce rated horsepower at any altitude (few rail lines are at exactly sea level.) All manufacturers of turbocharged locomotive engines have made this advantage apparent, time and again, over many years. Moreover, GE had a horsepower advantage over both the ALCO and EMD units.
At about the same time as the U23 was introduced, then, the 39 series appeared. This was a turbocharged locomotive with AC-DC transmission standard (AC-DC was optional on the U23, and later on the normally aspirated GP38 as well.) Electro-Motive now had the advantage of being able to offer a range of units, essentially, in terms of price and performance. Consider three basic configurations:
GP-38 normally aspirated, direct current
GP-38AC normally aspirated, alternator-rectifier
GP-39 turbocharged, alternator-rectifier
This gave EMD the ability to essentially be able to bid on any specification required by any railroad for units in this general range of performance. Were EMD to bid against the C-420 with the GP-38 it could point to lower maintenance costs. If compared to the U23 it could point to higher output ..and of course in any case, parts commonality with other EMD units sure to be finding their way onto the roads at that time (40 and 45 series units.)
Operationally it was easily possible to match the GP-38 with the SD-40 with almost identical minimum continuous speeds, and ordered properly, similar weight per axle. The GP-39 did have slightly higher power and Performance Control, but the important point was full output at any altitude and one might expect operation with other turbocharged units in these conditions; if that were true, then, the GP-30, GP-35, GP-40 and SD-45 all also had performance control which in all cases but the 30 lost much more than 300 HP at drag performance speeds (1000 HP in the case of the GP-40, as an example.)
Hopefully this is a little easier to understand than just the most basic comparison of number of cylinders and turbocharger or no.