In my opinion (as someone who doesn't weld, but does a lot of research on these topics); the difference should not cause any problem either way (60 or 70 degree included angle, vertical up or downwards progression).  You ought to have good access to put the root in with either joint configuration.

When welding downhand it is crucial to achieve a very close tolerance fit.  From what I've learned, this is the most important and often overlooked detail.

Charles

V,
If you look in API 5L Specification for Line Pipe, Paragraph 7.9.3 you will find that, unless otherwise specified, plain end pipe comes with a 30 degree bevel (+5/-0 degrees) and a 1/16" land (+/- 1/32").  This gives you a nominal 60 degree single v groove when fit up for welding.  This is referred to as a standard "API" bevel.  You can specify any bevel configuration and in the case of heavy wall pipe, many go to a modified J prep with a fairly low bevel angle (3-10 degree bevel angle).  Your choice will depend on many things, including pipe wall thickness, welding process, etc.

Hope this helps.