Which bike workout will get me better results?

Why not rotate between courses to mix it up.  Is your race flat or hilly?  Just because your course is flat, it doesn't mean some hills won't help...and vice versa.

what are you racing on?

Without going into zones and all that, the basic gist - practice on the kinds of conditions you'll race... that'll make you faster in the race itself.

x2 depends on the course you'd be racing.

Assuming you can recover from each, the one in which you do the most work.  If you ride shorter, ride harder.  If you ride longer, ride easier.  If you can handle it, ride longer and harder.  The terrain only matters in that it may force you to ride harder at times (i.e., whether you want to ride harder or not, you need to get up that hill).

Personally, when faced with an "either-or" choice in workouts, I choose to work my weakness.  On the bike, that means hills.

First I would say make sure you can cover the distance without question, if you fatigue towards the end, you need to keep working on your base, so the longer ride will probably prove more beneficial. 

If you are comfortable with the distance, start working in some hills or pushing the pace on the flats, as mentioned above, just because you are on a flat does not mean you cannot work harder than on hills, its all about gearing and boils down to power. 

Take the long ride for example.  You could start out slower for the first section and build through your ride focusing on getting up to race intensity.  This will get you used to your HR steadying out as you are racing. 

I'd offer a third alternative. I've found that I get the best bang for my time by incorporating two structured interval workouts into my routine. When I do the Carmichael Training Time Trial video workout of 3x 5 minute maximum efforts with 5 minutes rest in between followed by 12 minutes of a tempo interval (85% or so of max) I get significant improvement in my 40k tt times. I can then go out and enjoy my weekend rides on whatever route I feel like riding on the weekends.

Just a thought.

Well, you can always simulate hills by increasing your intensity on the flats. I look at my route based on the workout I am planning on doing. If I am riding jst a basic aerobic effort, I'll ride hills for some variabilty of that effort, if I am looking to do sustained aerobic effotsat a specific intensity or tempo/speed work, a flat course is much better.