Both your lenses.... 35/1.4 and 135/2... use a 72mm filter. Not a 77mm (unless you want to use a step ring and do without the lens hoods, which isn't a good idea if it can be avoided).
Tiffen does make some really good filters, but most you'll find are cheapos.
There are few really good variable ND... the Singh-Ray is probably the best and costs close to $400 US in the size you need. Forget the $25 Opteka, Neewer, etc... crappola. Forget the $75-100 Polaroid. More crappola. Not familiar with the Cameron or some of the other brands mentioned, but some of the sample images look pretty bad.... blotchy and uneven, maybe some color casts too.
Hoya, B+W, Marum, Heliopan, some Kenko and a few other quality brands of single strength ND filters will likely be the best deal... probably about $100 each, for multi-coated, high quality glass.
Lee's rectangular filters also come in P Cokin size, if interested in using graduated NDs too (which are better bought in rectangulars). The Cokin filter holders are relatively inexpensive. One down side to them, lens hoods are sort of limited... Cokin's are rigid modular (probably a single one would work for a 35mm lens, probalby would need three modules for the 135mm). But Lee makes an accordian type lens hood for the Cokin P holder, too. I think Cokin has been in bankruptcy recently, but would be suprised if no one bought it up and started making their line again (their filters are okay for occasional use or initial experimentation, but are also uncoated and pretty soft, easily damaged plastic).
The Lee filters are not multi-coated, though.