Ruby is not modern Smalltalk. It was inspired as much by Perl as by Smalltalk. In fact, one of the frequent gripes against Ruby is its annoying “Perlisms.”
Syntactic style is almost always the defining trait of a programming language. Ruby’s syntax is totally different from Smalltalk’s.
Pharo is more properly the modern Smalltalk.
In terms of performance, Pharo has made significant strides. Its Cog VM provides impressive performance gains. And if you need really high performance, GemStone/S is a fantastic option.
While Smalltalk has suffered from fragmentation in the past, I believe Pharo is well on its way to unifying the Smalltalk market. It is quickly becoming the one true Smalltalk. Pharo is easily the most actively innovating Smalltalk dialect. (It introduced the groundbreaking Glamorous Toolkit.)
Squeak and GNU Smalltalk are essentially stagnating. Dolphin is Windows-specific. Cuis is inconsequentially small. VisualWorks and VA Smalltalk are commercial dialects that aren’t favoured by open source supporters.
I am working extraordinarily hard to make Smalltalk relevant once again. Pharo leads the way.