The Roku isn't as flexible IMO nor as good sounding. Where did you get the idea that a squeezebox won't play MP3?
Here is an extract from the Slim Devices site for the SB3
Audio formats
Lossless Formats (Apple Lossless, FLAC, WMA Lossless)
"Bit-perfect" CD audio streaming, with reduced storage and bandwidth usage.
Approximately 2:1 compression ratio
Uncompressed formats (AIFF, WAV, PCM)
Supports raw pass-through of uncompressed PCM audio formats
Digital passthrough to S/PDIF for DTS
Compressed formats (MP3, AAC, Ogg Vorbis, MP2, MusePack, WMA)
MPEG decoding uses MAD software, widely regarded as the most accurate, most compatible MP3 decoder.
High accuracy 24-bit synthesis
Supports all MP3 data rates and sample rates, including VBR
May be streamed in PCM (raw), MP3 (transcoded), or FLAC (transcoded) formats
Sound Check and Replay Gain support for automatic volume control.
Some formats may require additional software installation (e.g. QuickTime), depending on platform.
Bryan