Still keep GW client?

I am installing GW2 and I have just linked my GW account no problem.

My question is weather I need both client?

I have one account with GW2 as well as the original GW's and expansions and I have another that has no GW2 but all the expansions of the first.

As in, can I play GW2 chars using my GW client? As both clients take up quite a lot of space.

Thanks.

as guildwars and guildwars2 are 2 different games you will need to have both exe files to play...you will not be able to log into gw1 with a gw2 client (or vise versa).

so yes, you will need to keep both

You do not need both clients to play one particular game. However, you need the gw1 client to play gw1, and the gw2 client to play gw2. The two games (gw1 & gw2) are completely different games, as I'm sure you've kinda noticed if you've played them at all. You cannot play gw2 using gw1, and vice-versa.
Yeh, they do take up a lot of space (gw.dat is like 4GB last I checked, and gw2.dat is like 15GB), but unfortunately there's no way around that. The .dat files store all the sound effects, textures, models, etc. from the games so that the game doesn't have to rely on the servers for that kinda basic game information.
If you have a portable hard drive, or an iPod with some free space, you might be able to offload the gw.dat file onto the ipod until you need it again. Unfortunately, I don't think this will work for gw2.dat, as the iPod seems to only accept files 4GB and smaller.