What cycling is best suited for marathon cross-training?

Question
I haven't cycled regularly for decades and now currently in training for my 6th marathon this fall. I like to consider picking it up again for my cross-training. Any thoughts of what the type of cycling, road or trail is best suited for this type of training and furthermore what kind of bike (brand and type) would you recommend? I have a budget between $500-1,000.00  

Answer
Ray, I am not sure what kind of shape you are in so I will assume you are in acceptable shape to be starting a training regimen.  I would say that either road or trail biking (or even Spinning) is good for cross training.  
Not knowing the rest of your training regimen, here are the goods and bads of each type:

Road - Positives: Better for training long, slow distance and getting used to 3-4 hour efforts required for marathons.  Very easy on your joints and very easy to practice feeding and keeping hydrated as you go.  Few bike maintenance issues.  Negatives: The danger of traffic, less fresh air, possible lack of hills to vary your efforts

Trail - Positives: Generally more hills for varying efforts (interval work comes with steep, short hills), fresh air, pleasant surroundings, away from traffic.  Negatives: More likelihood of minor injuries, more bicycle maintenance, harder to practice feeding and hydrating.

Spinning - Positives: Very easy on joints. Little chance of injury.  With a good instructor, you get a good interval workout. Easy cleanup, takes less time.  Negatives: No scenery, hard to find a good Spin instructor.

That said, the way most people train is to just do a ton of miles.  I believe that is the least effective way to train. Yes, you should have a long run each week, but if you do that too often, you will just run your body into the ground.  For the best all-around fitness, you should have a mixture of long, slow distance (LSD), medium faster distance, and intervals.  Biking can help best with making the long slow distance easy on your body and by pushing you in the intervals.  

As for a bike, the type depends most, in my opinion, on what you have easiest access to: roads or trails.  Remember that proper training for a long effort takes a lot of time and the less time you spend getting to the biking venue, the better.  Thus, you should buy a road or mountain bike to suit your most convenient training venue.  In Ohio, where I live right now, roads are more numerous than good mountain bike areas, so I ride the road more.  When I was in Utah, I rode only mountain bikes because I could go from home.

To answer your final question: $500-$1000 will get you into good, solid, entry-level road and mountain bikes from Specialized, Trek, Cannondale, Felt, Scott, Rocky Mountain and Kona.  I have recommended these brands to friends and these brands have never let me down.  Most important factor: The quality of the frame.  You can upgrade every other part.  

If you need more specific info, please give me some details on your workout plan and venues.  With a bit more detail, I can answer your questions more directly.