FILTER HELP NEEDED

[tatamkinophoto]

hi,
i have a Tamron 17mm-50mm f/2.8, a Sigma 30mm f/1.4 and a canon 55-250mm f/4-5.6 on a Canon 600D body.
I am about to purchase my first filters, Grad ND, ND and CPL but I don't think I can afford each filter for each lens.
I ruled out the zoom as I use it less often than the others. I thought of getting "square filters" and buy different mount ends for the Sigma (62mm) and the Tamron (67mm) to save a bit. Are square filters just as good as round ones?
On which lens it would be better to get the filters for if I only bought one filter size: "Tamron 17mm-50mm f/2.8" OR "Sigma 30mm f/1.4"?

Thanks for the help,
tatamkinophoto

 

[fjrabon]

get a filter for the biggest filter thread you have, they make relatively cheap adapters for the smaller filter threads.

 

[480sparky]

Get 67mm filters and a 67-62mm reducer.

Squares are better for Grad NDs as you can move them around... you're not fixed to the grad being across the center of the image.

 

[Mully]

Get a good glass filter it will last you a lifetime.... i have some old Tiffens that still work great. The screw on rings are not expensive and store easy. Cokin made so interesting filters years age that were durable plastic...I see them on ebay once in awhile.