The question the OP asks is "which one did you enjoy more?" Not which one is "better". It's impossible to say one is better than the other. Anyways here's my take on it..and keep in mind some of this is just opinion.
GT3 Pros-
Graphix seem more vivid.
The replay system in 3 is amazing, with multiple, spontaneous camera angles and 4 different replay modes.
Oversteer.
SS Route 11
Some folks would say Complex String as well.
I personally liked GT3's intro better
GT3's menu is also easier to navigate (opinion) and is quicker.
GT3 engines truly "break-in" with power
GT3 Cons-
GT3's Beginner League is way too easy, to the point of idiocy at times. I understand this is why it's called "Beginner League", but there aren't enough slow cars in GT3 for us to start the game with. Not for drivers like me who like a challenge at all times.
Way too monotonous after awhile. Not as many tracks for instance, so we wind racing the same ones over and over...
Not as many cars, either. GT3 is the only game in the series I got truly bored of for this reason, although I also understand
why there weren't as many cars
No used car lot. Searching for that perfect gem in the used lot is part of the fun. Every single GT has a UCL except GT3.
GT3 seemed "slower" overall, when racing.
GT3 also only has two driving views (bumper cam and an enormous, unusable chase cam)
GT3's tracks don't have as many irregularities and bumps on them, aren't as fun to drive for this reason.
GT3's races often feature one "jackrabbit" type car, which gets way ahead of everyone else, which creates a 1-sided challenge
GT3's races often feature stupid decisions with tires, so that during the Professional League, we start seeing cars coming in for pitstops after 3 laps...during a
7 lap race, for instance :facepalm #2:
All those MULTIPLE LAP RACES AT THE Test Course! :facepalm#3: No wonder we wound up calling it "Test Curse"
GT4 Pros-
Hundreds more cars (duh)
Tire monitor is on for ALL races. In GT3, it's only shown for races longer than 3 laps iirc.
Fuel
Weight balance in the tuning menu
Dozens more tracks (including more real-world locations)
B-spec racing is a neat feature for immersion, because it's like you've got this racing partner to help during endurances
Used car lot returns. Actually 3 used car lots.
Wing kits
Cars can lose their rigidity over time, as they roll over bumps and whatnot.
Tuning houses, which feature Stage 5 turbo systems & ORG suspensions.
A-spec features points system, which at least gives a reason for some drivers to detune their car's power and not overkill every race they enter.
GT4 has a "speedier" feel when racing, in comparison to 3.
GT4 also (sometimes) has more than just 1 car challenging us for the win (although some races in 4 still wind up becoming a "catch the jackrabbit" fest).
Added hood/roof cam, chase cam isn't so gigantic anymore (but it's still too big imo).
Off-road is more immersive, including snow tracks
Oh yea...PHOTOMODE! 👍
GT4 Cons-
HARDLY ANY OVERSTEER 🤦
NO DRIFTING in cars which
should oversteer & drift capability
More understeer than 3. This could be tolerated if we also had the realistic amount of oversteer to balance understeer out.
NO more Rome tracks, or SS Route 11, or Complex String
Graphix slightly inferior, not as vivid and dazzling as 3.
Replay system also took a nosedive, with less options and less modes.
More "generic-ness" with some cars. After driving dozens of models of different make & manufacturer, after awhile you wind up hearing the same soundbyte samples for certain engines.
No skidpad. If I recall, GT3 had an actual skidpad for testing. GT4 has Gymkhana, which I never really got into since we can't drift or toss cars as easily in 4 anyways.
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Which game is more enjoyable? If GT4 had more oversteer capacities, it would be a slam-dunk, but GT4 still wins slightly when all factors are considered. I spent way more time in 4, made more online friends, participated in online pursuits, had a more enjoyable time searching out GT4's hundreds of cars, and found 4 more immersive overall.