The underlying color space for processing in Adobe raw applications is a variant of ProPhoto RGB. So I'd (I do) simply render into ProPhoto RGB in 16-bit and move from there.
No printer can even print the entire color gamut of sRGB. But there are printers who's color spaces are shaped such that a big RGB Working Space is necessary so colors you can capture and output do not clip, hence again, ProPhoto RGB.
The Dano and Schewe video is needed a good one and I recommend you watch it.
Of, you can also watch this and try the testing process either with the supplied image or your own:
The benefits of wide gamut working spaces on printed output:
This three-part, 32-minute video covers why a wide gamut RGB working space like ProPhoto RGB can produce superior quality output to print.
Part 1 discusses how the supplied Gamut Test File was created and shows two prints output to an Epson 3880 using ProPhoto RGB and sRGB, how the deficiencies of sRGB gamut affect final output quality. Part 1 discusses what to look for on your own prints in terms of better color output. It also covers Photoshop’s Assign Profile command and how wide gamut spaces mishandled produce dull or oversaturated colors due to user error.
Part 2 goes into detail about how to print two versions of the properly converted Gamut Test File file in Photoshop using Photoshop’s Print command to correctly setup the test files for output. It covers the Convert to Profile command for preparing test files for output to a lab.
Part 3 goes into color theory and illustrates why a wide gamut space produces not only move vibrant and saturated color but detail and color separation compared to a small gamut working space like sRGB.
High Resolution Video: http://digitaldog.net/files/WideGamutPrintVideo.mov
Low Resolution (YouTube): https://www.youtube.com/watch?v=vLlr7wpAZKs&feature=youtu.be
Hi Andrew, in your video your recommend making prints using Perceptual and Relative rendering intents - which I have now done.
What should I be looking for -
or does it become a matter of personal preference?
The red in the "Relative" seems a little more intense than the "Perceptual", I see differing shades in the Grainger Rainbow....but, which one is "better"?
From a technical point of view, the top dark-blue Bill's Ball has some banding on both, but the "Perceptual"is worse...
Which of the two renderings do you prefer?

Many thanks for all your valuable input, it is very much appreciated!
William