As you use it there is no difference, but here is an example where you can see how it would matter

julia> test = Dict(:A => 1, :B => 4)
Dict{Symbol, Int64} with 2 entries:
:A => 1
:B => 4
julia> S(test)
ERROR: MethodError: Cannot `convert` an object of type Symbol to an object of type Int64
Closest candidates are:
convert(::Type{T}, ::Ptr) where T<:Integer at ~/git/julia/usr/share/julia/base/pointer.jl:23
convert(::Type{T}, ::T) where T<:Number at ~/git/julia/usr/share/julia/base/number.jl:6
convert(::Type{T}, ::Number) where T<:Number at ~/git/julia/usr/share/julia/base/number.jl:7
...
Stacktrace:
[1] setindex!(h::Dict{Int64, Int64}, v0::Int64, key0::Symbol)
@ Base ./dict.jl:373
[2] Dict{Int64, Int64}(kv::Dict{Symbol, Int64})
@ Base ./dict.jl:104
[3] convert
@ ./abstractdict.jl:525 [inlined]
[4] S(d::Dict{Symbol, Int64})
@ Main ~/.julia/packages/Parameters/MK0O4/src/Parameters.jl:505
[5] top-level scope
@ REPL[11]:1
julia> T(test)
T
d: Dict{Symbol, Int64}

since T will allow any type, just that the default is the same, but S will not allow any other type. If you know that you always want the field to be Dict{Int,Int} you probably want S.