All - Airey Baringer, an ActiveNetwork cycling super-freak offers up some sound bike purchasing info. It's fairly cerebral, but dead on.
The key to buying bicycles is to make sure that it meets your needs. The most expensive part of entry level to mid level road bikes are the components (shifters, pedals, chainrings, derailleurs, etc…). With that said, the frame on the cheaper Allez is better than the frame that I train and race on. I put about 200 – 250 miles on the bike per week, including commuting to work daily.
Thus, the frame on either bike will be a great investment that should last you for a while. The next consideration is components. The biggest question you need to answer is: how often you will be riding? Considering quality only, if you only ride 2-3 times per week on average, then the cheaper option should be fine as long as you take good care of the bike (keep it clean, regular tune-ups, etc…). If you ride more than that, you need to have higher quality components throughout the bike. The more expensive Allez option is at the tip of the iceberg for meeting this requirement, but it will do the job just fine as long as you take really good care of it.
Will this be your first road bike? Because I don’t know, I’m going to assume that it is. Based on this assumption, I can tell you that if you get hooked on this sport, you will most likely outgrow both of these bikes in the first couple years. If you don’t get completely hooked on the sport and just use it as an occasional recreational activity with a few races thrown in, then both of these bikes meet that need. Further, if you do get hooked on the sport, you can make either of these bikes last you longer than 2 years by updating the components one piece at a time. By that time, you will most likely know what you want and will be able to get it, so I’m not going to go into detail about this.
Based on all of the rambling mess above, I would say that you need to get the cheaper bike if you will be riding less than 2-3 times a year and you have no previous experience with road bikes.
Concerning the question of compact vs. double, you need to take into account where you will be riding, your current level of fitness, and how much effort you are willing to put into the bike to ride it well (yes, this can be done on 2-3 days of riding per week). A compact simply has smaller chainrings in the front. Thus, climbing may be easier. However, your top speed will suffer with a compact if you are on a flat course (as most triathlons are). Also, the gearing advantages provided by having a compact crankset can almost (but not quite) be attained by having a cassette (gears in the back) with a larger gear, like 27 teeth on the largest ring and a regular double crankset in the front.
So, you need to determine what kind of riding you will be doing more of (climbing or flats), your current level of fitness, and how willing you are to learn to ride the bike efficiently (involves reading and learning from more experienced riders). If you are fit, and don’t have to ride a lot of monster hills (think Palomar Mtn. @ 12 miles long) all the time, I would suggest a double. If you aren’t fit and do a lot of climbing, then you should get the compact. If you aren’t fit but do mostly flat riding, you should get the double. Also, the cassettes (gears in the rear) on both bikes are forgiving enough that you should be able to get up Lusk St. on your way to the office without any trouble using a double if you’re fit.
Most importantly, make sure that whatever you choose, the bike is fit to you. You should not be fit to the bike.