I'll add my 2 cents to the 'it depends' choir. My first sensei was a very experienced judo ka before taking up aikido, and he did beautiful ukemi and was a great aikido-teacher. No doubt his judo background acted as a sounding board for his aikido. The same is probably true for Nishio sensei and other japanese aikido-shihans. They can utilize judo and other budo experiences in their quest for 'their own aikido'.
I think however that it might help you develop good aikido, but it wont get you to good aikido (what ever that is) any faster. Also judo is different from dojo to dojo as far as i understand. Some judo organisations tend to focus on the competition aspect and this can have the unfortunate effect that people rely too much on muscle and physique than on technique. This is probably a bad tendency to bring into most aikido dojos.
So.. my answer is: if you had done judo for many years it would probably help you learning aikido, but since Aikido is what you want to learn - and you havn't - go practice aikido. Then take the odd classes or seminars with a judo sensei or someone who incorporates this in aikido some time in the future. you need to build a solid base for your aikido, but this base can be build from good aikido, and you will be able to go straight into learning instead of having to 'unlearn' first. I did Shotokan karate and Kendo for some years before aikido.. took me a while to change my mindset and the way I move. No doubt I have had some benefits from my background, but I would still wish I had started out in Aikido from the very beginning. Man I would have been good today then
Do what you want to do. Don't worry about cross-training as a prerequisite for doing good aikido. It is not. You can learn anything you need to learn for the next several decades by going to the right aikido dojos.