The differences between the various Fuji Roubaix models are mostly component choices as in things like the shifters and wheelsets that come with the bikes, some of those are small differences and some not so small.
The SL and 1.0 come with a full carbon fork as opposed to carbon fork legs bonded to an alloy steer tube. Not a deal breaker but the full carbon will be lighter and is a more expensive component. Realistically it's not the kind of thing you'll likely notice while riding the bike although many folks might try to convince you of the mythical properties of carbon pieces. They're nice, they're expensive, they're light but some of my favorite bikes have steel or alloy forks and or steer tubes so I wouldn't shell out a ton of cash for just a full carbon fork.
The 2.0 and 3.0 models are equipped with 9 speed drive trains which are less expensive than the 10 speed drive trains (number of rear cogs) on the 1.0 and SL models. Again it's not a make or break deal but the extra cogs are nice and more modern.
The wheelsets are different and the more expensive models have somewhat nicer wheels mounted but none of these bikes ships with radically high end wheels and they're all decent for the price. Besides wheels are really easy to upgrade down the road if you want to get something aero or fast.
Basically as you go up in price you get some nicer parts including wheels and front forks but any of these bikes would be very appropriate for training, long days in the saddle or even getting into racing.
In terms of geometry and 'aggressiveness' not all road bikes are built the same. Changes in things like wheelbase length, seat and head tube steepness, bottom bracket height, fore aft weight distribution, and front end designs impact overall feel and handling of the bike. Some bikes feel very quick, will initiate turns with just subtle shifts of weight and are designed to be ridden fast in situations like racing and fast group riding. These bikes tend to be very responsive to the rider's inputs whether it's feeling very responsive when a rider jumps up to sprint or quickness handling the bike in high speed pack riding or big mountain descending situations.
Other bikes are a tad more relaxed in their geometries can feel more stable can feel easier to ride in a straight line without a lot of attention or may feel easier to ride no hands for instance while eating or drinking on longer rides. In general these bikes might not feel as 'quick' though a good rider can make them go plenty 'fast' they may not dart in and out of holes in a big racing pack quite as quickly or not feel as quick to dive into a tight race corner at speed. More relaxed bikes often feel a bit softer over bumpy pavement or less than perfect roads but that often has more to do with tires and tire pressures than the bikes themselves.
Bottom line, not all bikes will feel the same when ridden so ride multiple models to feel this for yourself and see which style you really enjoy. You may not love a more relaxed bike or it may be just what you're after and make you feel confident about going out on long rides. No one can say but you so ride different bikes, different brands and different models in your general price range and buy the one that you really fall in love with.
-Dave
