Hello Amanduh,
To quickly answer your question there isn't a lot of difference between the two, with the 400 cc implant being 14% larger than the 350 cc implant. To give you an idea what that means, when women are unhappy with their size after breast augmentation and want a revision surgery to go larger or smaller, I recommend a change of at least 25% to 30% to see a visible difference.
The best way to communicate your aesthetic goal to your surgeon is to bring in photographs of people who's proportions you like. In addition, your surgeon should be able to 'size' you in his office, a process where you place sizing implants into a bra and cover with a top to assess how you would look. The combination of the photos and sizing gives both you and the surgeon a good idea of how you want to look (your aesthetic goal).
Now having said that, it would be impossible to recommend an implant size for you without a thorough examination of your anatomy, so I can not comment about one implant versus another. Information obtained from an examination includes breast base width and height, tissue elasticity, tissue thickness, and asymmetries of the breast or ribcage. All these variables come together, along with your personal aesthetic goals, allowing the surgeon to make the best choice for you.
Keep in mind that choosing an implant that is inappropriately large for your anatomy is the most common mistake that plastic surgeons make (at the behest of their patients, who's only concern is the cup size after surgery); one that is costly for the patient and will most likely lead to unexpected issues, dissatisfaction with the results, and frequently, a reoperation.
Pretty, long lasting results after breast augmentation surgery are gotten from an appropriately sized implant (based on the patient's anatomy, nothing to do with subjective measures like proportion), that has either a medium or low profile, and is placed under the pectoralis major muscle.
Best of luck!