I also consider the 07 Sentra before I bought Mazda 3 (I actually also consider the Civic and tC, but that is another story). Like many have said, you test drive both of them and get familiarize with all the features then you decide which one you like better. If possible test drove both of them back to back. That is what I did last time. Here is my comment for each car.
Mazda 3:
+ The 2.3L engine felt more powerful
+ Handling is better
+ Brake is excellent
+ I like the interior design better, ie the tachometer blue and red color looks very nice and also the design on the dashboard.
+ Exterior design looks better than the 07 Sentra in my opinion
+ Heated seat for the GT (I can't comment on this because I didn't get the GT nor did I test drove the GT)
+ Has HID headlight on the GT (Sentra doesn't have it as far as I remember)
- No bluetooth for cell phone (might not be relevant to you)
- MPG not as good as the 07 Sentra (at least on paper)
- Keyless entry system not as good as the Sentra
- Road noise is more than Sentra
- Ride might be a little bumpy for some, but it's a trade off for the excellent handling
- Less interior space and trunk space (for sedan, not sure about the hatch vs Sentra)
As for reliability, so far many reviews say the 3 is good.
I also like that there is an active forum for Mazda 3, just in case you have questions, you can ask around. I think there should be one too for Sentra, but I haven't found one that is as big as this forum (most that I've seen is more for the previous Sentra, Versa, or the altima/maxima. Maybe because the 07 Sentra is very new). I suggest you also post on Nissan forum too to get their opinion.
The 07 Sentra:
+ Larger interior space and trunk space (at least if you compare it to the Mazda 3 sedan)
+ The oversize text and numbers on the center LCD (for radio and trip computer) are very easy to read.
+ CVT is very nice, no shift shock at all but you might need to get use to it first because it feels a little weird initially. (The Mazda 3 auto is also very smooth, but I think CVT is slightly better. If you are looking for manual, then this is irrelevant)
+ Many little things that I think they design it well such as adjustable cup holder in the center, the cd holder near the visor, large glove box (if you think Mazda 3's glove box is large, check the 07 sentra)
+ I think it has less road noise than the 3
+ I like their keyless entry and start without taking the key out of your pocket
+ Bluetooth for your cell phone
- Engine is not as powerful as the 3, but I think it is enough for daily driving
- Handling is not as good as the 3, I felt more body roll in Sentra
- Engine noise is more noisy (compare to the 2.3L Mazda 3) when you accelerate, when idle they are comparable
- The interior design doesn't look as nice as the 3, but I think it is more practical
- No heated seat
Nissan reliability should be good too I think. Although they are having problems with the previous 2.5L engine on Sentra and Altima. The current engine is newly design engine. You should probably know more about Nissan than me because you own a Nissan.
Now having said that, I like both cars but in the end I choose the 3 because I think the handling is better, the engine is smoother/more quiet and I felt the 3 (2.3L engine) has more power than the Sentra. I also like the styling inside and outside better on the 3. I know that I will miss the keyless entry on the Sentra and also the bluetooth, but I think it is better than missing the handling of the 3. I guess it really depends on what features you are looking for. I think Sentra has more tangible features than the 3, but the 3 drives better. The 3 I think is designed to be a "sporty" compact car while the Sentra is designed to be more on the comfort side.
If you already decide on the Mazda 3, I think any of the color looks good on the 3. I chose the aurora blue. But I also like the galaxy gray a lot. Like other said, go to dealer and take a look in person. Good luck with your car search.
Any input would be greatly appreciated.
