Body design of the two pens are quite similar. They are both plastic body pens with the exception of the metal clip and pen tip. Both are technical pens that create strokes with uniform width. Both are available with line widths from 0.1 to 1.0.
The green pen at the bottom is the Rapidograph. Rotring is not the only company that makes the Rapidograph pen, the other is Koh-I-Noor. I'm not sure if the Rapidographs from Koh-I-Noor are similar to Rotring's though.
Performance of both Rapidograph and Isograph are the same. Ink flow is the same too.
The Isograph can be dismantled completely and easily. With the Rapidograph, I was not able to dismantle the air channel (the one fused to the tip) to remove the pin that's inside. So cleaning the Rapidograph may take longer time.
If you have problem removing the Rapidograph air channel, you can screw it to the back of the pen's body and pull it out gently. It's likely to get stuck because of ink or dried ink. But be careful not screw it so tight that you can't remove it later.
Rapidograph is meant to be used with disposable ink cartridges while the Isograph has a cartridge that can be easily refilled.
If you refill the Rapidograph's ink cartridge by inserting the Rotring ink bottle's funnel, the ink is going to block the opening. If the ink cartridge opening is blocked, no air can come out and no ink can go in.
Hence the better way to refill the Rapidograph is with a blunt needle and syringe. It's more economical to refill with your own ink than having to buy disposable ink cartridges.
The Isograph ink cartridge has a larger opening and you can easily squeeze ink into it with the Rotring ink bottle.
Overall, the Isograph is easier to maintain and refill. If you're looking for a refillable technical pen, I would recommend the Isograph over Rapidograph.
You can buy the pens individually or in sets. If you're buying individually, I suggest getting a few line weights, the more popular ones being 0.3, 0.5 and 0.7. If you need more variety then 0.1 and 1.0. The thinner the line, the more maintenance is required. If the ink dries in the small pen tip, it is going to clog and be very difficult to unclog.
Regular maintenance should see these pens last for years. So while each pen is expensive, they are still economical in the long run, especially when compare them to disposable technical pens.