If you're looking at compact cars with four doors, you could do a lot better than the two you picked.
The Toyota Yaris was recently rated as one of the worst small cars you could buy by Consumer Reports, possibly the most belligerent Japanese-loving magazine out there. Only the Chevrolet Aveo was ranked lower.
The Sentra did quite a bit better. It's also quite a bit larger car than the Toyota Yaris, more in the same size category as the Toyota Corolla.
If you're looking at Yaris-sized cars, look at these models:
Ford Fiesta
Honda Fit
Hyundai Accent
Kia Rio
Nissan Versa
Scion XD
Suzuki SX4
If you're more after a Sentra-sized cars look at these models:
Ford Focus
Honda Civic
Hyundai Elantra
Kia Soul
Kia Spectra
Mazda 3
Saturn Astra
Scion XB
Toyota Corolla
Volkswagen Jetta
Volkswagen Rabbit
Out of the above models, my personal favorites are the Suzuki SX4 for the smaller models and theMazda 3 and Volkswagen Jetta and Rabbit for the larger models.
All of these models should be under $350 a month, fully loaded.
I'd say your best bet would be one of the Hyundais, Kias, or the Suzuki, as they come with the most standard equipment. The Volkswagens are also very well optioned, and have the most luxurious, comfortable, and stylish interiors, but they will be the most expensive in this group. Still, they're cheap for what you'll get.
I hope this has answered your question. If you have any more, please feel free to ask.