These two birds somewhat closely resemble each other, but the Sedge Warbler is russet-brown above, the feathers with dark centres, the pale buff eyestripe is very clearly defined, and the underparts are buffish white; the Reed Warbler is more olive on the upper parts, the feathers having no dark centres, the underparts are more inclined to buff, and the eyestripe is nearly obsolete.
Source: wiktionary