Based on the Epson specifications:
Feature | v300 | v500
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Native DPI | 4800 | 6400
Optical Density | 3.2 | 3.4
Slide Support | 35mm | 35mm & Medium Format
Reliability | 10,000 cycles | 36,000 cycles
Features of the v300:
- Achieve exceptional clarity and detail with 4800 x 9600 dpi optical resolution
- Scan 35mm slides and negatives with ease using the built-in Transparency Unit (for beautiful enlargements up to 13x19)
- Restore the color to old, faded photos with one touch
- Do more with one-touch document scanning — send e-mails or create PDFs
- Quickly copy documents and photos for archiving purposes
- Enjoy remarkable versatility, plus fully automatic scanning
- Scan books, photo albums and 3D objects with high-rise, 180-degree lid
- Enjoy more efficient operation with exclusive ReadyScan™ LED technology — no warmup time and fast scanning speeds
- Rest easy with earth-friendly LED technology — no mercury included and lower power consumption
- Enhance your images with photo editing and creative software included
- Scan text with amazing clarity — Office Mode delivers sharp scans, even if text is on colored paper or a double-sided document
Features of the v500:
- Create extraordinary enlargements — create 13" x 19", 17" x 22" or larger sizes from film with 6400 dpi
- Remove the appearance of dust and scratches from film — DIGITAL ICE™ for Film
- Scan slides, negatives and medium-format panoramic film — built-in Transparency UnitRestore faded color photos with one touch, using Easy Photo Fix®
- Achieve greater productivity — greater productivity with the energy-efficient, earth-friendly LED light source — no warmup time, faster scans, lower power consumption
- Take your photos further — Adobe Photoshop Elements included, to help edit and enhance your digital imagesGet brilliant, true-to-life colors with amazing, 48-bit color depth
- Quickly scan multiple documents — optional Automatic Document Feeder
- Convenient connectivity — Hi-Speed USB 2.0 included
As far as I can tell, the v500 doesn't necessarily do more film in one go. It does, however, support more film types, including 6x12 panoramic medium format. A more important feature of the v500, I think, is its native resolution. For film, high scanning resolution is important in generating a digital image that can be printed at large sizes. If you only intend to print at 4x6 or 5x7 sizes, the v300/v330 would probably be enough. If you need to scan medium format film, or print at larger sizes up to 17x22, the v500 would be necessary.