+1 to the suggestion of going with the "densely planted" look with plastic plants. It's harder to vacuum up the crud with more plants packed in there, but it really helps keep things looking more natural than a sparse 'scape will, and the fish seem to appreciate the extra cover. Also, modifying the short "mats" into irregular shapes as suggested above can make a huge difference in helping make things look natural.
As far as silk vs plastic, it's personal choice. Silk tends to look more natural at first, but it does get gummed up, turn frayed, etc. You can extend the life of them by soaking them in cleaning products and then rinsing/treating thoroughly, but this can have catastrophic results if done incorrectly. Also, no matter how well you keep them clean, eventually they typically start to fray too much and need to be replaced.
Plastic is much simpler to clean and doesn't require as much rinsing/treating in the rare cases you have to use cleaning products on them. It also doesn't usually need replacing over time unless you're a bit too rough on scrubbing them and accidentally rip pieces off (which can usually just be superglued back on). It does tend to look more fake unless you go for the really spendy ones though.