I am truly not in a position to rate both, but I have both systems and both games.
I have had Forza for a much longer time, and recently went out of my way to upgrade to the oddly marketed and sold forza ultimate edition. I only say this because I am in the US and could not find the game at any retail locations. I had to eventually order it on Amazon. The reason I had to have this version is here nor there, because I had just about all of the DLC. I did want the extra cars and was willing to pay for them after my older version fetched a good price to offset the new copy.
Here are my thoughts on the games.
I think that Forza is a more open varied game. Making money is easier, driving has more options should you be a car person, but not a big driver. It is also a great game should you like to paint or build cars a specific way. To me it is a more "fun" game based on the options, variety, lack of all cars being "premium" looks to start the game, not having to play with horrible cars for a long time to progress, the game moving more smoothly and not feeling like such a grind to progress.
GT5.
In some aspects, I think that the game looks better, nearly photo realistic. That being said, to get to that point where you have the "premium" cars, it is very much a grind. I love car games, but I have always hated this about GT games. The first sections of the game are truly hurtful to the overall experience. They are truly "racing simulation" games. No matter how much you love cars, getting to the point where you have money and fun in the game is sort of a chore if thats not your thing. I love the game, the graphics, and the overall quality, but that barrier to entry to truly appreciate or love the game the way that it should be loved as an overall package is very high. You really have to work for a long time before you get good cars, and many of the beginning races/cars in the game have a jaggie/ps2 look to them that is inexcusable for a game that was being made for this long. Though there may be reasons for this, I cannot give Sony or Polophony a pass on this because as a game that I spent $60 on, it should be a complete game, and I do not feel that was about a huge number of the intro cars. I think that people will get turned off by the quality of some of the cars and the grind that it is to make progress in the game. that being said, I have had the same issue with GT games in the past, and I have reached out to my online race buddies to help me progress and make it past this. Finally I think that the progression system is somewhat broken. By this I mean, early in the game I will enter a race with a car that I think is great for that race, yet I keep losing. I look at the cars entered and then I check their prices. All too often there are cars in these races that I could not afford till later in the game and spec-wise price themselves out of the race/class so to speak. By this I mean, I may have spent maybe 150 to 200k on a car. I look at the other cars and I see cars competing and winning that cost 15+ million dollars. To me there is an issue there. Taking this scenario into account, I then spend a chunk of money on upgrades to make the best car available to me competitive to win these early races in the game and win a lot less money than I could win, am stuck with a car that is only good for specific races, and am in a position to not have enough money to repeat the process to win the subsequent races. These are my personal gripes with the game and the series.
That all being said, I love GT5 given these flaws and for the money you cannot go wrong. As NBA 2k11 is quoted as being a "love letter to MJ and basketball fans," GT5 with its 1100 cars is nearly the same thing to car fans. The sheer number of cars, events, quality of the tracks, detail given to the actual driving experience make this game a "racing game RPG of the highest order. I do not think that anyone would or could finish this game in a short period of time. Personally I love cars and love to collect them in the games. I hate giving them up, so having so many potential cars is mind blowing.
I am not sure what your budget is, but if it is low, then get Forza Ultimate Collection, you will not be disappointed.
If you are a middle of the pack person in regards to cars and games, then get Forza Ultimate Collection and a good wheel. Again, you will be a happy camper.
If you have a bit more money and are on the fence with these games, shop around for price and try to get both either used or on a good deal.
If money is not that big of a deal, then get both, and consider which one you would make better use of with a steering wheel.
I think both games are close in some regards, but far in others.
I love the cars, but am not so much on the racing part. Forza makes it easier for me to make choices to play the game I want. I can put on assists that cost me money, but make the game more accessible than it ever would have been. I can thus "not grind" collect cars and have money for cars when I need them. The ability to race following the season format or enter any race you want is also huge to me. It makes the game and the cars available where they night not have been otherwise. GT5 takes a different approach with their leveling and strict guidelines for entering races or even buying cars. Again I think that the decision breaks down to what you want out of the game and how you play. I cannot specifically say which is better, but I can say because I have both games that the experience is different and caters to different gaming types.They are close, but different. Finally, I am not a huge "race" kind of person, so other posters may be able to answer better as to which game technically recreates the experience better.
I truly hope this helps as this is the most typing I have done outside of school and work